Can I ever be natural?

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  • Опубликовано: 3 окт 2024
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Комментарии • 238

  • @GiornoYoshikage
    @GiornoYoshikage Год назад +201

    The interesting thing is that 7 is the value of infinite nested root √(42+(√42+√(42+√(...)))). Actually, if you replace 42 with x*(x-1) for x>1, the limit will be exactly 'x'.

    • @JeanYvesBouguet
      @JeanYvesBouguet Год назад +10

      Excellent observation! Making 42 a really special number 😊

    • @Neodynium.the_permanent_magnet
      @Neodynium.the_permanent_magnet Год назад +20

      @@JeanYvesBouguet Not so special. Take any √(m+√(m+x)) and do m = x² - x
      For instance x=5, √(m+√(m+5)) take m = 25 - 5 ...

    • @vetbaitednv
      @vetbaitednv Год назад +6

      @@Neodynium.the_permanent_magnet 42 is certainly a special number though

    • @Rougarou99
      @Rougarou99 Год назад

      @@Neodynium.the_permanent_magnet I wonder if there is a name for numbers that fit this pattern. I.E. some pattern n(x), where n(7)=42, n(5)=20,...

    • @Neodynium.the_permanent_magnet
      @Neodynium.the_permanent_magnet Год назад +2

      @@Rougarou99 It works for any N

  • @dlevi67
    @dlevi67 Год назад +40

    Case 2 can be even more quickly ruled out through parity. (4n² + 29 is odd, (2n+2)² is even)

  • @aaademed
    @aaademed Год назад +82

    There is much simpler way to find n from 4n^2 + 29 - is square
    Lets 4n^2 + 29 = k^2
    then
    (k-2n)(k+2n) = 29 | 29 is a prime number so 29 = 1*29
    then we got system of linear equations:
    k = 2n + 1 => k = 15
    k = 29 - 2n => n = 7
    The rest goes the same

    • @doodle1726
      @doodle1726 Год назад +1

      This is good

    • @swenji9113
      @swenji9113 Год назад +9

      Yes it's a shame not to use the fact that 29 is prime, given the opportunity

    • @anshumanagrawal346
      @anshumanagrawal346 Год назад

      Ikr

    • @lewsouth1539
      @lewsouth1539 Год назад

      Yeah, that's how I did it.
      For any odd prime p, the only natural numbers whose squares differ by p are (p ± 1)/2.
      Guess I'll delete my comment now....

  • @davewpearson
    @davewpearson Год назад +5

    42 the answer to life the universe and everything !!!

  • @galveston8929
    @galveston8929 Год назад +87

    Another way: let sqrt(m+7)=a and sqrt(m+a)=b, then m+7=a^2 and b^2-a=m, replacing m from the latter to the former eq we have: b^2-a+7=a^2, after multiplying by 4 and adding 1 to both side, we can rearrange the last eq into (2a+1)^2-(2b)^2 = 29. Hence (2a+2b+1)(2a-2b+1)=29 which is prime therefore it can only be factored into 1 and 29. 2a+2b+1 is the bigger factor thus 2a+2b+1=29 and 2a-2b+1=1, solving for a and b we get a=b=7 thus m=b^2 - a =42.

    • @averagegamer9513
      @averagegamer9513 Год назад +6

      You wrote the the difference of squares wrong, it should be (2a-2b+1)(2a+2b+1) as you wrote later.

    • @galveston8929
      @galveston8929 Год назад +7

      thanks for noticing the obvious typo. Just fixed it.

    • @leif1075
      @leif1075 Год назад

      @@galveston8929 why would you or anyone think to multiply by 4 at all though or multiply by anything even?

    • @bktreesdoesmc8957
      @bktreesdoesmc8957 Год назад +3

      @@leif1075 from a^2 + a = b^2 - 7, you can complete the square in terms of a and obtain (a+1/2)^2=b^2+29/4, which motivates the multiplication by 4 to obtain a difference of integer squares.

    • @galveston8929
      @galveston8929 Год назад

      @@leif1075 that's part of completing the squre. When you do this many times, then you'll do it by heart.

  • @jacobgoldman5780
    @jacobgoldman5780 Год назад +5

    Case 3: We have 4n^2+29=(2n+3)^2 so 4n^2+29=4n^2+12n+9 so 12n=20 which does not provide another solution.

  • @normanstevens4924
    @normanstevens4924 Год назад +19

    Surely it's simpler if you notice that if 4n^2 + 29 is a perfect square, p^2 say, then p^2 - (2n)^2 = 29 and therefore (p - 2n)(p + 2n) = 29. So as p + 2n is larger of the two factors we must have p + 2n = 29 and p - 2n = 1. Thus p = 15 and n = 7.

    • @joshcarter1018
      @joshcarter1018 Год назад +4

      While there is nothing wrong with what you have written, and indeed (at least as far as I'm aware) it is a simpler solution, you should probably have specified that your last line is only possible because 29 is a prime (and hence irreducible).

  • @marcushendriksen8415
    @marcushendriksen8415 Год назад +4

    What a fun problem and solution! I couldn't help modifying the problem and giving it a go myself!

  • @criskity
    @criskity Год назад +2

    Finally found the question to the answer to life, the universe, and everything!

  • @speedsterh
    @speedsterh Год назад +5

    Super easy to follow explanations, thank you Michael

  • @petraveryanov2572
    @petraveryanov2572 Год назад +3

    My solution: sqrt(m + 7) should be integer, so m = k*k - 7, then all we need is k*k - 7 + k to be a square. We can check all k < 7 and for all k > 7 this cannot be square since k * k < k*k - 7 + k < (k + 1)*(k + 1). So k = 7 is only solution and m = 42

  • @AnAverageItalian
    @AnAverageItalian Год назад +68

    The 3rd case for n doesn't work out, because
    4n²+29=(2n+3)²
    4n²+29=4n²+12n+9
    As always, the 4n² cancels out, so we get that
    12n=20
    n=20/12=5/3 which isn't an integer, so it doesn't count here

    • @ariel_haymarket
      @ariel_haymarket Год назад

      Came here with the same thought and glad to see someone else had come to the same conclusion

    • @Grassmpl
      @Grassmpl Год назад

      Easier just to check mod 3.

    • @AnAverageItalian
      @AnAverageItalian Год назад

      @@Grassmpl oh really? I'm not that familiar with modulo stuff, could you show me?

    • @afa12345
      @afa12345 Год назад

      @@AnAverageItalian you can substract 3 on the right equation to get 4n^2 +29 = (2n)^2 (mod 3), then eliminate the n we have 29 = 0 (mod 3) which is simply wrong

    • @sumongus
      @sumongus Год назад

      @@afa12345 subtract*

  • @matthieubrilman9407
    @matthieubrilman9407 Год назад +2

    Another solution : Let m and q be such that m+7=q²
    (1) m + √(m+7) = q² - 7 + q = q² + 2q + 1 - (q+8) and hence m +√(m+7) < (q+1)²
    (2) Also, if q > 7 then m > 42 hence m + √(m+7) > m + 7 = q².
    From (1) and (2) we get that if q >7 then q² < m + √(m+7) < (q+1)² and hence √(m + √(m+7)) cannot be an integer.
    After that, all that is left is to check the values of q from 1 to 7.

  • @andy-kg5fb
    @andy-kg5fb Год назад +7

    05:30 we can assume (2n)²+29=k² for natural k.
    So we get 29=(k+2n)(k-2n)
    Which as k and n are positive integers, and 29 is prime, there is only one possibility,
    k+2n=29
    K-2n=1
    so we get n=7.

  • @guidomartinez5099
    @guidomartinez5099 Год назад +13

    For 4:50 onwards, 4n^2 is already a perfect square, and the consecutive squares are always some odd number away, so 4n^2 and 4n^2 + 29 could be 196 and 225 (the largest possible answers), giving n=7; or smaller squares separated by an (odd) number of these "steps". But since 29 is prime, there are no others. (Then find m as you did.)

    • @jursamaj
      @jursamaj Год назад

      I don't think 29 being prime gives the result you think it does. The 'step' between squares wouldn't be 29 divided by the step size. Instead, for a single step difference, you look for the smaller square to be (29-1^2)/(2*1)=7. For the next odd step, it would be (29-3^2)/(2*3), then (29-5^2)/(2*5), etc.

    • @guidomartinez5099
      @guidomartinez5099 Год назад +2

      @@jursamaj My reasoning is that if they are 3 odd numbers away, then call them k-2, k, and k+2, so the difference is 3k, which cannot be since 29 is prime. Same for any other odd number.

    • @lewsouth1539
      @lewsouth1539 Год назад +1

      @@guidomartinez5099 Exactly right. If p is prime, p = h^2 - k^2 = (h + k)(h - k) gives h + k = p & h - k = 1 as the only possible factors.

  • @iyadgaber6780
    @iyadgaber6780 8 месяцев назад

    sure a nice way to get the day started, talking for myself here. I got a friend to hop on discord and we watched it together first thing in the morning. Keep up the good work!

  • @s4623
    @s4623 Год назад +4

    7:27 it's much easier to move the square term to the right and factor because after you factor the difference of square it has to be even [ (2n+2-2n)(2n+2+2n) = 2(4n+2) ] and you have an odd number on the left. Same applies to the (2n+3) case; (2n+3-2n)(2n+3+2n) = 3(4n+3) which is divisible by 3 but 29 is not divisible by 3. Also slightly easier for (2n+1) because you don't have to multiply it out as one of the factor becomes one when you do the difference of squares: (2n+1-2n)(2n+1+2n) = 1(4n+1) = 29 so n = 7

  • @dugong369
    @dugong369 Год назад +1

    Or, let y=sqrt(m+7). Then m=y^2 -7. Then the original expression converts to sqrt(y^2-7+y). Therefore y^2+y-7 must be a perfect square. But this is always less than (y+1)^2=y^2+2y+1, and for y>7, it is greater than y^2 (therefore in between 2 consecutive squares and not a perfect square). So we only have to check seven values of y in y^2+y-7 to see if any is a perfect square, and get y=7. We know m=y^2-7, so m=42.

  • @johnyjohnjohnson1317
    @johnyjohnjohnson1317 Год назад +1

    thanks for explaining why the "+" case goes wrong

  • @Terence3184
    @Terence3184 Год назад +1

    set sqrt(m+7)=n and m=n^2-7 then sqrt(m+sqrt(m+7))=sqrt(n^2+n-7) between n and n+1

  • @NotoriousSRG
    @NotoriousSRG Год назад +194

    Hey folks - I edited this video. Let me know if you think the sound is better since I tried to clean it up.

    • @goodplacetostop2973
      @goodplacetostop2973 Год назад +8

      Oh so that’s why it wasn’t live at the usual time 😂

    • @NotoriousSRG
      @NotoriousSRG Год назад +10

      Nah. This was edited a couple days ago lol

    • @NotoriousSRG
      @NotoriousSRG Год назад +14

      I don’t control when it goes up i just edit lol

    • @roryisatall1
      @roryisatall1 Год назад +5

      There is a bit of a thing everytime he says "s", but other than that sounds great

    • @NotoriousSRG
      @NotoriousSRG Год назад +10

      @@roryisatall1 thank you for that feedback. I can try on future videos to fix that.

  • @CRGreathouse
    @CRGreathouse Год назад +1

    You need m + sqrt(m+7) to be a square, so in particular it has to be a natural number, so m+7 is a square. Call it m+7 = s^2 so m = s^2 - 7 and the goal is for s^2 + s - 7 to be a square. This is between (s+1)^2 and s^2 unless s is small; in particular either s^2 + s - 7 = s^2 or s^2 + s - 7

  • @thbb1
    @thbb1 Год назад +1

    found m=42 by noticing that: m+7 must be a perfect square, otherwise m+sqrt(m+7) can't be a perfect square. Thus, m must be divisible by 7 and adding 7 to it has to be a perfect square too, which leaves 6*7=42 as the only possible solution. This gives me m=42 and n=7 as the only possible solution.

  • @pseudo_goose
    @pseudo_goose Год назад +1

    You can find the 42 solution pretty quickly, by making the substitution sqrt(m+7)=7. That makes the outer radical equal to the inner radical, and since the inner is a perfect square from that equation, the outer is also. From there it is easy to solve for m=42

    • @honourabledoctoredwinmoria3126
      @honourabledoctoredwinmoria3126 Год назад

      Yes. All the math is only needed to prove there is never another solution. It's pretty trivial to guess that m = 42 is a solution.

  • @DavesMathVideos
    @DavesMathVideos Год назад +6

    Once again it would seem that one can find a solution, m=42 by inspection but the difficulty lies in proving it's the only solution.

    • @Khaim.m
      @Khaim.m Год назад

      Yes, I saw that solution immediately by thinking "what if the inner sqrt is also 7" but it's not obvious that's the only solution.

    • @MrTrollo2
      @MrTrollo2 Год назад

      But that also wasn't the question in the video title

  • @chaosredefined3834
    @chaosredefined3834 8 месяцев назад

    As another approach... We manage to get that we need 4m^2 + 29 to be a perfect square. Therefore, we have some k such that 4m^2 + 29 = k^2. Let p = k - 2m. Therefore, k = 2m + p. So we have 4m^2 + 29 = (2m + p)^2. Expanding the RHS, we get 4m^2 + 29 = 4m^2 + 4mp + p^2. This means that 29 = 4mp + p^2. But the right hand side is clearly divisible by p. So, 29 has to be divisible by p. That means that p is either -29, -1, 1 or 29. Using an argument similar to what Michael did, we can eliminate -29, -1 and 29. Therefore, the only candidate is p = 1, so we have 4m^2 + 29 = (2m + 1)^2. Solve as Michael did.

  • @goodplacetostop2973
    @goodplacetostop2973 Год назад +11

    12:58

    • @MyOneFiftiethOfADollar
      @MyOneFiftiethOfADollar Год назад +4

      In an age of specialization, your have procured a micro-niche. "The human time stamp of a signature phrase"

  • @thatdude_93
    @thatdude_93 Год назад +3

    Title sounds like you’re having an existential crisis

  • @kennethvalbjoern
    @kennethvalbjoern Месяц назад

    Cool. Nice trick with the bounding inequalities.

  • @kostasch5686
    @kostasch5686 Год назад

    At the start you squared the quantity n^2-m. After that you should have kept in mind that m

  • @demenion3521
    @demenion3521 Год назад +2

    my intuition at the start told me that it's unlikely for there to be 2 perfect squares that fit into the radicals in the presented form, so my guess was that the inner root must be the same as the outer root and hence sqrt(m+7)=7 which directly gives the solution m=42. of course it's not a prove that there are no other solutions, but it felt unlikely to me

  • @Axacqk
    @Axacqk Год назад

    5:31 "Rewriting" something as something entirely different just because the inequality will still hold is probably the most confusing trick in the entirety of algebra.

  • @0114mercury
    @0114mercury Год назад +2

    If you say that m = n^2-7 (which is must be), the algebra becomes much simpler.

  • @marcushendriksen8415
    @marcushendriksen8415 Год назад +1

    This problem inspired me to try another problem: given a natural number c, find natural numbers m and n such that root(m + root(m + c)) = n. Following the same basic method that Michael showed us, I worked out a really quick way to do it: find the odd factors of (4c+1); these will be called "k values". For each value of k, calculate (4c+1-k^2)/4k; each natural number that results will be a possible "n value" (note that if 4k is greater than half the value of 4c+1-k^2, the result cannot be natural). For each possible n value you can get up to 2 possible m values, which are calculated using m = 1/2×(2n^2+1±root(4n^2+4c+1)). In the case where you end up with more than 1 ordered pair, you have to check them manually.

  • @elyades2480
    @elyades2480 Год назад

    Thanks for the video !!! Very much enjoyed

  • @abrahammekonnen
    @abrahammekonnen Год назад +1

    I really liked the discussion on the extraneous solutions, and that 3rd case shouldn't give you a solution.

  • @ZekeRaiden
    @ZekeRaiden Год назад

    Perhaps less elegant, but a convenient process I went through:
    We know that if (m+7) is not a perfect square, then the whole equation cannot be a natural number. Hence, the possibility space for m is restricted to m = q^2-7 for nonnegative integers q. We can exclude q=0,1,2 because those values would make sqrt(m+7) negative and thus make n imaginary. The first handful of valid m values are 2, 9, 18, 29, 42, 57, 74, which we can use to show both that a solution exists, and that that solution is unique. These are associated with the q values 3, 4, 5, 6, 7, 8, 9.
    We get the following results from these options, simplifying the sqrt(m+7) parts to just the associated q value.
    sqrt(2+3) = sqrt(5), invalid (4 shy of 3^2=9)
    sqrt(9+4) = sqrt(13), invalid (3 shy of 4^2=16)
    sqrt(18+5) = sqrt(23), invalid (2 shy of 5^2=25)
    sqrt(29+6) = sqrt(35), invalid (1 shy of 6^2=36)
    sqrt(42+7) = sqrt(49) = 7, valid!
    sqrt(57+8) = sqrt(65), invalid (1 above 8^2=64)
    sqrt(74+9) = sqrt(83), invalid (2 above 9^2=81)
    Each time, you add 1 to the difference between m+sqrt(m+7) and q^2. This gap grows linearly. However, in order for there to be at least two solutions, there would need to be _quadratic_ growth in that gap. As a result, 7 is the only q value that works, and thus m=42 is the only valid integer solution.

  • @properlol1340
    @properlol1340 Год назад

    You can also simply notice 29 is odd, so it is the sum of 2 consective numbers, which means it is thr difference of their squares, namely 14 and 15. From their, since 14=7x2, its square id 4×7^2, which lends 7.

  • @kamilnalikowski3186
    @kamilnalikowski3186 Год назад

    11:04 . There should not be any self checking if you were cautious enough to see that n^2 - m >=0 before squaring at 1:27.

  • @mathpuzzles6352
    @mathpuzzles6352 Год назад

    Good video, it is a fine solution! Thanks for making it!

  • @at7388
    @at7388 Год назад +1

    U did a good job, Notorious.

  • @noahdavis3663
    @noahdavis3663 Год назад

    the answer seems so obvious once you find it out. it could work for any number. if the problem was sqrt(m + sqrt(m + 6)) then it would be 30 since 6^2 - 6 is 30

  • @johnny_eth
    @johnny_eth Год назад

    I just sat the thumbnail and immediately recognized the common sub expression
    a=sqrt(m+7) and b=sqrt(m+a), and hence assumed a=b and thus m+7=49, just like in the power tower n=x^x^x^...^n.
    To be more strict, and don't assume anything, just expand further
    a^2=m+b => a^2-b=m
    b^2=m+7 => b^2-7=m
    Then match both
    a^2-b=b^2-7
    And the symmetry is evident, hence a=b=7

  • @Dalton1294
    @Dalton1294 Год назад

    Both case 2 and case 3 have solutions if m is allowed to be a rational number and that √(m+√(m+7)) is also allowed to be rational

  • @Valkonymous
    @Valkonymous Год назад

    My solution:
    square both sides gets: n^2 = m + sqrt(m+c) (1). Set b=sqrt(m+c) (2)
    solve (2) for m gets b^2 - c = m. (3)
    Substitute (2) and (3) into (1) gets n^2 = b^2 - c + b.
    Two solutions for n to be natural:
    First b=c (4) gets n^2 = c^2. Substituting (4) into (3) gets m=c^2 - c.
    Second b=-c-1 (5) gets n^2 = c^2. Substituting (5) into (3) gets m=c^2 + c + 1.
    Thus at c = 7, m=42 or 57.
    Check sqrt(42+7) = +/- 7 => 42 +7 = 49 or 42 - 7 = 35. 49 is a perfect square so n is a natural number.
    sqrt(57+7) = +/- 8 => 57 + 8 = 65, 57-8 = 49. 49 is a perfect square so n is a natural number.
    Edit, I guess that was supposed to only be a principle root so 42 is the only solution since b must be positive. So the full solution for m is m = {c^2 - c, if c>=0; c^2 + c + 1, if c < 0}

  • @heartache5742
    @heartache5742 Год назад

    of course it's that number
    the answer to life and the universe and everything

  • @EternalLoveAnkh
    @EternalLoveAnkh Год назад +1

    If we replace 7 with an arbitrary x, then m = x^2 - x.
    RJ

  • @RexxSchneider
    @RexxSchneider Год назад

    That's a lot of case checking, and I hate case checking if I don't have to.
    Let n = √(m + √(m+7)), and note that only the square root of a perfect square can be a natural.
    If m is a natural, then for m+√(m+7) to be natural, we must have √(m+7) is natural, so (m+7) is a perfect square, call it a^2 (where a ∈ ℕ). So m = a^2 - 7. Substitute for m:
    n = √(m + √(m+7)) = √(a^2-7 + a). Square both sides:
    n^2 = a^2 + a - 7. Now rearrange to a quadratic in a:
    a^2 + a - 7 - n^2 = 0
    a = (-1 ± √(1 + 28 + 4n^2))/2 = (√(4n^2 + 29) - 1) / 2. For a to be natural, 4n^2+29 must be a perfect square, call it b^2 (where b ∈ ℕ). So a = (b-1)/2.
    So b^2 - 4n^2 = 29 or (b+2n)(b-2n) = 29. That has one solution among the naturals, where (b+2n) = 29 and (b-2n) = 1, since 29 has only one positive factorisation, and (b+2n) > (b-2n).
    That sole solution yields b = 15 and n = 7. Hence a = (b - 1)/2 = 7 and therefore m = a^2 - 7 = 49 - 7 = 42.
    Check: √(42 + √(42+7)) = √(42 + √49) = √(42 + 7) = √49 = 7 = n, as calculated. There are no more solutions and no cases to check.

  • @moonshine8233
    @moonshine8233 Год назад +2

    I read the title and really thought this was going to be about something other than math.

  • @danpost5651
    @danpost5651 Год назад

    I came up with 42 as a valid solution just by thinking about what was under the radicals. If m+7 was a perfect square, then if I can make the inner radical, sqrt(m+7), equal to "7", then the outer radical would be the same thing -- sqrt(m+"7"). Therefore, m+7 should be 49, which makes m equal to 42.

  • @pinguino55h40
    @pinguino55h40 Год назад

    Assume m + 7 is a square n^2, then m = n^2 - 7. So the expression becomes sqrt(n^2 + n - 7), thus n^2 + n - 7 needs to be a perfect square. Notice the distance between two squares (n + k)^2 - n^2 is exactly 2kn + k^2 for any integer k. Since n^2 is a square, then n - 7 needs to be a number of the form 2kn + k^2 so that n^2 + n - 7 is a square => n = (k^2 + 7)/1 - 2k, which we need to be an integer. Notice we have a trivial solution when k = 0. We can simplify with the substitution k = (1 - j)/2 => n = (j^2 - 2j + 29)/4j. Since n is an integer, 4j divides j^2 - 2j + 29. Now, j^2 - 2j + 29 = 0 (mod j) => 29 = 0 (mod j), thus j = 29 because it is prime, which works and yields k = -14 => n = 7 => m = 42.

  • @konraddapper7764
    @konraddapper7764 Год назад

    Great to see different aproaches giving the Same result
    My solution was the Same Up to the Point where you calculate
    m = 1/2( 2*n^2 +1 +- squrt(4n^2 +29)
    Than used the Data that only Numbers
    i

  • @krisbrandenberger544
    @krisbrandenberger544 Год назад

    For the 3rd case, you get n=5/3 which does not give a solution.

  • @General12th
    @General12th Год назад

    Hi Dr. Penn!

  • @FTR0225
    @FTR0225 Год назад

    You can be whatever you want if you set your mind to it

  • @samuelbodansky2
    @samuelbodansky2 Год назад

    m=-3 is a solution in the integers

  • @pietergeerkens6324
    @pietergeerkens6324 Год назад

    Surely it's basic number theory that the sum of successive gnomons between two perfect squares is divisible by the number of gnomons being summed!
    It's just an arithmetic series of consecutive odd numbers and:
    case 1) If of an odd number of terms, then equal to the middle term times the number of terms; and
    case 2) if of an even number, say 2k, of terms, then equal to the 4k/2 times average term and again divisible by 2k.
    Thus 29 being prime, the only perfect squares that it can separate are 14^2 and 15^2.

  • @KingGisInDaHouse
    @KingGisInDaHouse Год назад

    I just guessed x=42 by looking at the inner radical and seeing what can make it a square.

  • @kianushmaleki
    @kianushmaleki Год назад

    Beautiful

  • @rialtho_the_magnificent
    @rialtho_the_magnificent Год назад +4

    42, always a good 'guess' I guess......

  • @paca5507
    @paca5507 Год назад

    11.33 "...42. A pretty popular number on the internet..." Just beautiful.

  • @juandesalgado
    @juandesalgado Год назад

    42 was easier to find by taking m+7=n^2 and m+n=q^2, then equating n=7=q to satisfy both equations. Of course that says nothing about finding all solutions.

  • @youtubenutzer4028
    @youtubenutzer4028 Год назад

    You can also prove it simpler:
    Let b = sqrt(m+7) => m = b^2-7 and for sqrt(m+sqrt(m+7)) to be natural we need sqrt(m+7) to be natural.
    So if m is a solution to our problem then b = sqrt(m+7) is natural and we have sqrt(b^2-7+b) is a natural number in other words b^2+b-7 is a square.
    But for all b > 7 we know that b^2 < b^2+b-7 < b^2+2b+1 = (b+1)^2. So that we must have b 2 since b = sqrt(m+7).
    So we check for b = 3,4,5,6,7 and find that b^2+b-7 is only a square if b = 7.
    Lastly we have to check if m = b^2-7 = 42 is really a solution of our problem and indeed sqrt(42+sqrt(42+7)) =7.
    Thus m = 42 is the only solution.

  • @datguiser
    @datguiser Год назад

    I just thought: what if we get sqrt (m+7) to equal 7, that way you know the outside sqrt will also be 7. Therefore, 7^2 - 7 = 42. Answer is 42.

  • @truejeffanderson
    @truejeffanderson Год назад +1

    If x = √(m+√(m+c)), then is it always true that natural x = c?
    Update: there's some recursion. Can we use:
    if x = √(m+x) then
    x = √(m+√(m+x))
    if we knew that the initial condition in the first sentence was always true, then the problem can be reduced to: 7 = √(m+7)
    Where then m is easy

  • @jeffcieslak5115
    @jeffcieslak5115 Год назад

    If you generalize it "backwards" - for any Natural n such that root(m + root(m + c)) = n, there are infinitely many integer pairs (m,c) that can satisfy the equation, and for each value of m, c=(n^2-1)^2-m. And (0,n^4) is always a solution, as well. The maximum value of m is n^2. When c=7, there is precisely one m (42) producing one n (7), as demonstrated - but for, say, c=17, there are two pairs that produce different n values: (19,17]) = 5 and (272,17) = 17. c=8 is the lowest positive integer with multiple m and n solutions.

  • @lool8421
    @lool8421 Год назад

    just by looking at it, i just figured out that sqrt(m+7) could be literally the same as sqrt(m+sqrt(m+7)) and therefore sqrt(m+7) = 7
    sometimes you might have some weird math intuition, but it's better to always check stuff regardless

  • @alxjones
    @alxjones Год назад

    We can get m = 42 pretty much by inspection. After all, if m + 7 is a perfect square, then sqrt( m + 7 ) = k and we ask which k makes m + k a perfect square, which we already know 7 is an answer. To make k = 7 we just need m + 7 = 7^2, or m = 42. Of course, this doesn't prove it's the only solution!

  • @MrJdcirbo
    @MrJdcirbo Год назад +1

    We now have the question to the answer...

  • @theartisticactuary
    @theartisticactuary Год назад +2

    Have you just discovered the ultimate question of life, the universe and everything? We've had the answer for a while but have been struggling to work out what the question was.

    • @stereodude016
      @stereodude016 Год назад +1

      Nice reference to the hitchhiker's guide to the galaxy saga.

  • @end41r53
    @end41r53 Год назад

    well basically you square the 7 from the equation and substract it and get 42

  • @CTJ2619
    @CTJ2619 Год назад +1

    haha 42 “pretty popular number on the internet”

  • @wesleydeng71
    @wesleydeng71 Год назад

    m = k^2-7 -> k^2+k-7 = a^2 -> (2k+1+2a)(2k+1-2a) = 29 -> k = 7, m =42.

  • @becomepostal
    @becomepostal Год назад

    Wow it looks like it was easy!

  • @justmarvin4926
    @justmarvin4926 Год назад

    Extending on the solution, for any integer k, k * (k - 1) is a solution to m where sqrt(m + sqrt(m + k)) has to be a natural number. So if k = 7, we get 42 as the value of m. If k = 8, we get 56...

  • @abrahammekonnen
    @abrahammekonnen Год назад

    Very true 42 is a very popular number on the internet.

  • @methodiconion8523
    @methodiconion8523 Год назад

    I'm sure this video goes into something interesting and important, but 42 jumped at me as an obvious hypothesis.

  • @Hazelpy
    @Hazelpy Год назад

    I find this problem very fascinating.
    I observed that the answer can be found very simply, actually
    If √(m + √(m + 7)) has to be a natural number, and m also has to be natural, then (m + 7) has to be equal to 7^2. √(m + 7) has to result in 7 so that the following √(m + 7) can be a perfect square.
    Thus, 7^2 - 7 = m, which also happens to give us our neat little 42 answer. :D

  • @filippochi143
    @filippochi143 Год назад +2

    The actual answer to the title is: “Dude what are you talking about, i is always imaginary”.

    • @lewsouth1539
      @lewsouth1539 Год назад

      Exactly what I thought-of course; it's only logical.

    • @filippochi143
      @filippochi143 Год назад

      @@lewsouth1539 LLAP, other Nimoy.

  • @luisisaurio
    @luisisaurio Год назад

    Sqrt (m+7) must be rational. If not m+sqrt(m+7) is irrational and therefore non squared, so m+7=n^2.
    n^2+n-7=k^2 -> n^2+n-(7+k^2)=0. We can finish from here just like the video or using the DOQ=Prime argument.

  • @ЕшгинРамильоглыМагеррамов

    Привет, я сейчас в 11 классе и мы считаем интегралы в уме(не шутка). Очень интересное видео, спасибо. Вы решали сборник задач Демидовича по матанализу?

  • @joaozin003
    @joaozin003 Год назад

    I already guessed 42 at the start, because sqrt(42+7)=7, which repeats!

  • @truejeffanderson
    @truejeffanderson Год назад

    Find m for y:
    y = √(m+√(m+7))
    Let x be natural where:
    x = √(m+x)
    Substitute right side into itself:
    x = √(m+√(m+x))
    If we let x be 7 we have the equation:
    7 = √(m+√(m+7))
    Depending on the existence of natural:
    7 = √(m+7)
    49 = m + 7
    m = 42
    Verify y is natural when m = 42:
    y = √(m+√(m+7))
    y = √(42+√(42+7))
    y = √(42+7)
    y = 7
    A solution for m = 42

  • @paca5507
    @paca5507 Год назад

    MPenn is just David Lynch of mathematics; the only distinction is MPenn gives solutions...

  • @anon6514
    @anon6514 Год назад

    When we get to the "difference of squares equals 29" bit,
    you can try writing 29 as the sum of consecutive odd numbers
    and discover there's only one way to do this:
    One summand:
    29 = 29
    Three summands:
    13 + 11 + 9 = 33 > 29
    11 + 9 + 7 = 27 < 29
    Five summands:
    11 + 9 + 7 + 5 + 3 = 35 > 29
    9 + 7 + 5 + 3 + 1 = 25 < 29
    Seven summands:
    13 + 11 + 9 + 7 + 5 + 3 + 1 = 38 > 29
    Therefore only 14^2 and 15^2 differ by 29

  • @sameerk7151
    @sameerk7151 Год назад +1

    Just came here to see if my solution is right I solved it in 20 secs just by THUMBNAIL 💀
    Proud to be an Aisan

  • @carlosdaniellamasbarcenas7244
    @carlosdaniellamasbarcenas7244 Год назад

    Let u= sqrt(m+7). u must be a natural because m is a natural number too. Then m=u^2-7. So the problem will turn into sqrt(u^2+u-7) = x, where x belongs to N.
    Then x^2=u^2+u-7. But this implies that:
    x^2-u^2=u-7 or (x+u)(x-u)=u-7
    If x>= u, then u >=7, but x+u>u-7 , So equality must be satisfied and u=7=> m=42.
    If x

  • @XanderOwen24
    @XanderOwen24 Год назад +1

    I cant prove it, but it seems for natural numbers A, where A > 0,the only natural number solution to sqrt(m+sqrt(m+A)) is A(A-1). So it generalizes real nicely I think

  • @Erik-in8fh
    @Erik-in8fh Год назад

    I just wrote down every square number, its root and itself minus his root and then you see that there is an interception at and only at 7 with m equals 42.

  • @Douae1111
    @Douae1111 5 месяцев назад

    really nice

  • @ere4t4t4rrrrr4
    @ere4t4t4rrrrr4 Год назад

    I'm not sure if you mentioned it, but because of the 1/2 factor, this only works when sqrt(4n² + 29) is odd (which, incidentally, is the case for n = 7)

    • @columbus8myhw
      @columbus8myhw Год назад

      sqrt(4n^2+29) will never be an even integer since it's the square root of an odd number. It will either be odd or irrational.

  • @oscarbizard2411
    @oscarbizard2411 Год назад

    Me instantly trying 42 out and going yessssss

  • @renatomagretti7938
    @renatomagretti7938 Год назад

    I immediately said 42 from the top of my head, but i didn't know how to prove it

  • @mego111
    @mego111 Год назад +2

    57 is also a solution Sqrt(64)=+/- 8

  • @steve2817
    @steve2817 Год назад

    m = n^2-7 (n >0)
    m+sqrt(m+7) = n^2+n-7 = (n+k)^2
    If 3 = 7, k >= 0
    n = (k^2+7)/(1-2k)
    k = 0 because n > 0
    n = 7, m = 42
    (Is there an error in this proof?)

  • @Terence3184
    @Terence3184 Год назад +1

    sqrt(n^2+n-7)=n to get the only solution n=7 m=7^2-7=42

  • @pow3rofevil
    @pow3rofevil Год назад

    Very nice

  • @MeNowDealWIthIt
    @MeNowDealWIthIt Год назад

    Case 3: 4n^2+29=(2n+3)^2
    =4n^2+12n+9
    20=12n
    n is not natural, so no solution there.

  • @cicik57
    @cicik57 Год назад

    wow so this exists only with infinite nested roots like solve √m+n = n and then use nested in √m+√m+... = n

  • @Sesquipedalia
    @Sesquipedalia Год назад +3

    guys idk what these symbols are. why do we need to find men?