Posted by **ant** on Friday, June 4, 2010 at 6:32pm.

What number is used to complete the square for the equation x^2 + 5x = 0?

a) 25

b) 6.25

c) 2.5

d) 10

d

- algebra -
**MathMate**, Friday, June 4, 2010 at 7:00pm
(d) is not correct.

When we need to complete the square of an expression, we examine the coefficients of the x² and x terms.

To complete the square of a general quadratic equation

ax²+bx+c

we first rewrite it as:

a(x² + (b/a)x + c/a)

to make the coefficient of x² equal to 1.0 inside the parentheses.

After that, we regroup the terms as

a( (x+(b/2a))² + c/a - (b/2a)² )

Therefore the number to complete the square is b/2 if a=1.

Post if you need further explanations.

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