If using the method of completing the square to solve the quadratic equation x^2+15x+25=0x
2
+15x+25=0, which number would have to be added to "complete the square"?

x = -15/2 ± (1/2)*5√5

Step-by-step explanation:

I will assume that you meant x^2+15x+25=0.

To complete the square, take half of the coefficient of x, square the result, add this square into the equation after '15x,' and then subtract the square from the result:

x^2+15x+25=0  =>  x^2 + 15x + (15/2)^2 - (15/2)^2 + 25 = 0.

Rewrite the first two terms as (x + 15/2)^2 - 225/4 + 100/4 = 0, or

                                                  (x + 15/2)^2 - 125/4 = 0

Taking the square root of both sides, we obtain the solutions:

                                                    x + 15/2 ±√[125/4], or

                                                    x = -15/2 ± (1/2)*5√5