Find the Vertex Form f(x)=-x^2-8x-20

$f (x) = - x^{2} - 8 x - 20$

Step 1

Write $f (x) = - x^{2} - 8 x - 20$ as an equation.

$y = - x^{2} - 8 x - 20$

Step 2

Complete the square for $- x^{2} - 8 x - 20$ .

Use the form $a x^{2} + b x + c$ , to find the values of $a$ , $b$ , and $c$ .

$a = - 1$

$b = - 8$

$c = - 20$

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{- 8}{2 \cdot - 1}$

Simplify the right side.

Cancel the common factor of $- 8$ and $2$ .

Factor $2$ out of $- 8$ .

$d = \frac{2 \cdot - 4}{2 \cdot - 1}$

Move the negative one from the denominator of $\frac{- 4}{- 1}$ .

$d = - 1 \cdot - 4$

Rewrite $- 1 \cdot - 4$ as $- - 4$ .

$d = - - 4$

Multiply $- 1$ by $- 4$ .

$d = 4$

Find the value of $e$ using the formula $e = c - \frac{b^{2}}{4 a}$ .

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = - 20 - \frac{{(- 8)}^{2}}{4 \cdot - 1}$

Simplify the right side.

Simplify each term.

Raise $- 8$ to the power of $2$ .

$e = - 20 - \frac{64}{4 \cdot - 1}$

Multiply $4$ by $- 1$ .

$e = - 20 - \frac{64}{- 4}$

Divide $64$ by $- 4$ .

$e = - 20 - - 16$

Multiply $- 1$ by $- 16$ .

$e = - 20 + 16$

Add $- 20$ and $16$ .

$e = - 4$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- {(x + 4)}^{2} - 4$ .

$- {(x + 4)}^{2} - 4$

Step 3

Set $y$ equal to the new right side.

$y = - {(x + 4)}^{2} - 4$