Find the Axis of Symmetry f(x)=-(x+9)(x-21)

$f (x) = - (x + 9) (x - 21)$

Step 1

Write $f (x) = - (x + 9) (x - 21)$ as an equation.

$y = - (x + 9) (x - 21)$

Step 2

Rewrite the equation in vertex form.

Complete the square for $- (x + 9) (x - 21)$ .

Simplify the expression.

Apply the distributive property.

$(- x - 1 \cdot 9) (x - 21)$

Multiply $- 1$ by $9$ .

$(- x - 9) (x - 21)$

Expand $(- x - 9) (x - 21)$ using the FOIL Method.

Apply the distributive property.

$- x (x - 21) - 9 (x - 21)$

Apply the distributive property.

$- x \cdot x - x \cdot - 21 - 9 (x - 21)$

Apply the distributive property.

$- x \cdot x - x \cdot - 21 - 9 x - 9 \cdot - 21$

Simplify and combine like terms.

Simplify each term.

Multiply $x$ by $x$ by adding the exponents.

Move $x$ .

$- (x \cdot x) - x \cdot - 21 - 9 x - 9 \cdot - 21$

Multiply $x$ by $x$ .

$- x^{2} - x \cdot - 21 - 9 x - 9 \cdot - 21$

Multiply $- 21$ by $- 1$ .

$- x^{2} + 21 x - 9 x - 9 \cdot - 21$

Multiply $- 9$ by $- 21$ .

$- x^{2} + 21 x - 9 x + 189$

Subtract $9 x$ from $21 x$ .

$- x^{2} + 12 x + 189$

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

$a = - 1$

$b = 12$

$c = 189$

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 $d = \frac{b}{2 a}$ .

$d = \frac{12}{2 \cdot - 1}$

Simplify the right side.

Cancel the common factor of $12$ and $2$ .

Factor $2$ out of $12$ .

$d = \frac{2 \cdot 6}{2 \cdot - 1}$

Move the negative one from the denominator of $\frac{6}{- 1}$ .

$d = - 1 \cdot 6$

Multiply $- 1$ by $6$ .

$d = - 6$

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 $e = c - \frac{b^{2}}{4 a}$ .

$e = 189 - \frac{12^{2}}{4 \cdot - 1}$

Simplify the right side.

Simplify each term.

Raise $12$ to the power of $2$ .

$e = 189 - \frac{144}{4 \cdot - 1}$

Multiply $4$ by $- 1$ .

$e = 189 - \frac{144}{- 4}$

Divide $144$ by $- 4$ .

$e = 189 - - 36$

Multiply $- 1$ by $- 36$ .

$e = 189 + 36$

Add $189$ and $36$ .

$e = 225$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- {(x - 6)}^{2} + 225$ .

$- {(x - 6)}^{2} + 225$

Set $y$ equal to the new right side.

$y = - {(x - 6)}^{2} + 225$

Step 3

Use the vertex form, $y = a {(x - h)}^{2} + k$ , to determine the values of $a$ , $h$ , and $k$ .

$a = - 1$

$h = 6$

$k = 225$

Step 4

Since the value of $a$ is negative, the parabola opens down.

Opens Down

Step 5

Find the vertex $(h, k)$ .

$(6, 225)$

Step 6

Find $p$ , the distance from the vertex to the focus.

Find the distance from the vertex to a focus of the parabola by using the following formula.

$\frac{1}{4 a}$

Substitute the value of $a$ into the formula.

$\frac{1}{4 \cdot - 1}$

Cancel the common factor of $1$ and $- 1$ .

Rewrite $1$ as $- 1 (- 1)$ .

$\frac{- 1 (- 1)}{4 \cdot - 1}$

Move the negative in front of the fraction.

$- \frac{1}{4}$

Step 7

Find the focus.

The focus of a parabola can be found by adding $p$ to the y-coordinate $k$ if the parabola opens up or down.

$(h, k + p)$

Substitute the known values of $h$ , $p$ , and $k$ into the formula and simplify.

$(6, \frac{899}{4})$

Step 8

Find the axis of symmetry by finding the line that passes through the vertex and the focus.

$x = 6$

Step 9

Find the Axis of Symmetry f(x)=-(x+9)(x-21)

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