Solve Using the Quadratic Formula x^2-x-72=0

$x^{2} - x - 72 = 0$

Step 1

Use the quadratic formula to find the solutions.

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

Step 2

Substitute the values $a = 1$ , $b = - 1$ , and $c = - 72$ into the quadratic formula and solve for $x$ .

$\frac{1 \pm \sqrt{{(- 1)}^{2} - 4 \cdot (1 \cdot - 72)}}{2 \cdot 1}$

Step 3

Simplify.

Simplify the numerator.

Raise $- 1$ to the power of $2$ .

$x = \frac{1 \pm \sqrt{1 - 4 \cdot 1 \cdot - 72}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot - 72$ .

Multiply $- 4$ by $1$ .

$x = \frac{1 \pm \sqrt{1 - 4 \cdot - 72}}{2 \cdot 1}$

Multiply $- 4$ by $- 72$ .

$x = \frac{1 \pm \sqrt{1 + 288}}{2 \cdot 1}$

Add $1$ and $288$ .

$x = \frac{1 \pm \sqrt{289}}{2 \cdot 1}$

Rewrite $289$ as $17^{2}$ .

$x = \frac{1 \pm \sqrt{17^{2}}}{2 \cdot 1}$

Pull terms out from under the radical, assuming positive real numbers.

$x = \frac{1 \pm 17}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{1 \pm 17}{2}$

Step 4

The final answer is the combination of both solutions.

$x = 9, - 8$