What number would be added to both sides in order to complete the square when solving x^2+8x=9?
a.16
b.18
c.64
d.81

You can figure this out if you just look at (x+a)^2 = x^2 + 2ax + a^2. See, the constant term, a^2, is the coefficient of x (2a), divided by 2 (2a/2=a), and then squared (a^2) 

So you equation has 8x. 8/2 = 4, and 4^2 = 16. So the answer is A.