Question

Which number completes the square?
X^2-20x+?

A. -100
B. -10
C. 10 <--- my answer
D. 100

Answer 1

$$

\Large{ x^2 + bx + \color{red}{(\frac b2)^2} = ( x + \frac b 2 )^2

\\ \therefore \\

x^2 + (-20)\cdot x +\color{red}{(\frac{-20}{2}) ^2} = ( x + \frac{-20}{2})^2

}
$$

Answer 2

Ok so its -10

Answer 3

don't forget to square it :)

Answer 4

100?

Answer 5

yes