# What can you add to x^2-9x to get a perfect square trinomial?

May 12, 2018

$20.25$

#### Explanation:

In a perfect square, the equation is $a {x}^{2} + b x + c$, where $\left\mid b \right\mid = 2 a \sqrt{c}$. Therefore, to find $c$, plug in $a$ and $b$.

$\left\mid b \right\mid = 2 a \sqrt{c}$

$\left\mid - 9 \right\mid = 2 \cdot 1 \cdot \sqrt{c}$

$9 = 2 \cdot \sqrt{c}$

$4.5 = \sqrt{c}$

$20.25 = c$