We have X+Y=5.and next equation is XY=25.From the second of XY=25. You. Have X=25/Y. Substitute this in X+Y=5. You get 25/Y+Y=5. The next step is 25+Y^2,=5Y.this is Y^2-5Y+25=0. Quadratic equation in Y. So two roots. On getting each value substituting for Y in 25/X=Y. You get X value.
We have X+Y=5.and next equation is XY=25.From the second of XY=25. You. Have X=25/Y. Substitute this in X+Y=5. You get 25/Y+Y=5. The next step is 25+Y^2,=5Y.this is Y^2-5Y+25=0. Quadratic equation in Y. So two roots. On getting each value substituting for Y in 25/X=Y. You get X value.
General solution of: x + y = a, xy = b
x, y = [a ± √(a² - 4b)]/2
EDIT
In this case a = 5 and b =25. so that x, y = [5 ± √(5² - 4·25)]/2 = 5 (1± √3i)/2
Wait what is value of X and Y then?
who said i is -1 ??? how you got there ?