Find the Axis of Symmetry f(x)=x^2+15x+3

$f (x) = x^{2} + 15 x + 3$

Step 1

Write $f (x) = x^{2} + 15 x + 3$ as an equation.

$y = x^{2} + 15 x + 3$

Step 2

Rewrite the equation in vertex form.

Complete the square for $x^{2} + 15 x + 3$ .

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

$a = 1$

$b = 15$

$c = 3$

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

$d = \frac{15}{2 \cdot 1}$

Multiply $2$ by $1$ .

$d = \frac{15}{2}$

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

$e = 3 - \frac{15^{2}}{4 \cdot 1}$

Simplify the right side.

Simplify each term.

Raise $15$ to the power of $2$ .

$e = 3 - \frac{225}{4 \cdot 1}$

Multiply $4$ by $1$ .

$e = 3 - \frac{225}{4}$

To write $3$ as a fraction with a common denominator, multiply by $\frac{4}{4}$ .

$e = 3 \cdot \frac{4}{4} - \frac{225}{4}$

Combine $3$ and $\frac{4}{4}$ .

$e = \frac{3 \cdot 4}{4} - \frac{225}{4}$

Combine the numerators over the common denominator.

$e = \frac{3 \cdot 4 - 225}{4}$

Simplify the numerator.

Multiply $3$ by $4$ .

$e = \frac{12 - 225}{4}$

Subtract $225$ from $12$ .

$e = \frac{- 213}{4}$

Move the negative in front of the fraction.

$e = - \frac{213}{4}$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form ${(x + \frac{15}{2})}^{2} - \frac{213}{4}$ .

${(x + \frac{15}{2})}^{2} - \frac{213}{4}$

Set $y$ equal to the new right side.

$y = {(x + \frac{15}{2})}^{2} - \frac{213}{4}$

Step 3

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

$a = 1$

$h = - \frac{15}{2}$

$k = - \frac{213}{4}$

Step 4

Since the value of $a$ is positive, the parabola opens up.

Opens Up

Step 5

Find the vertex $(h, k)$ .

$(- \frac{15}{2}, - \frac{213}{4})$

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 $1$ .

Cancel the common factor.

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

Rewrite the expression.

$\frac{1}{4}$

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.

$(- \frac{15}{2}, - 53)$

Step 8

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

$x = - \frac{15}{2}$

Step 9

Find the Axis of Symmetry f(x)=x^2+15x+3

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