Find the Roots (Zeros) f(x)=x^2-18x-4

$f (x) = x^{2} - 18 x - 4$

Step 1

Set $x^{2} - 18 x - 4$ equal to $0$ .

$x^{2} - 18 x - 4 = 0$

Step 2

Solve for $x$ .

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Substitute the values $a = 1$ , $b = - 18$ , and $c = - 4$ into the quadratic formula and solve for $x$ .

$\frac{18 \pm \sqrt{{(- 18)}^{2} - 4 \cdot (1 \cdot - 4)}}{2 \cdot 1}$

Simplify.

Simplify the numerator.

Raise $- 18$ to the power of $2$ .

$x = \frac{18 \pm \sqrt{324 - 4 \cdot 1 \cdot - 4}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot - 4$ .

Multiply $- 4$ by $1$ .

$x = \frac{18 \pm \sqrt{324 - 4 \cdot - 4}}{2 \cdot 1}$

Multiply $- 4$ by $- 4$ .

$x = \frac{18 \pm \sqrt{324 + 16}}{2 \cdot 1}$

Add $324$ and $16$ .

$x = \frac{18 \pm \sqrt{340}}{2 \cdot 1}$

Rewrite $340$ as $2^{2} \cdot 85$ .

Factor $4$ out of $340$ .

$x = \frac{18 \pm \sqrt{4 (85)}}{2 \cdot 1}$

Rewrite $4$ as $2^{2}$ .

$x = \frac{18 \pm \sqrt{2^{2} \cdot 85}}{2 \cdot 1}$

Pull terms out from under the radical.

$x = \frac{18 \pm 2 \sqrt{85}}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{18 \pm 2 \sqrt{85}}{2}$

Simplify $\frac{18 \pm 2 \sqrt{85}}{2}$ .

$x = 9 \pm \sqrt{85}$

The final answer is the combination of both solutions.

$x = 9 + \sqrt{85}, 9 - \sqrt{85}$

$x = 9 \pm \sqrt{85}$

Step 3

The result can be shown in multiple forms.

Exact Form:

$x = 9 \pm \sqrt{85}$

Decimal Form:

$x = 18.21954445 \dots, - 0.21954445 \dots$

Step 4