Solve 7x^2+14x+24=9

$$7 x ^ { 2 } + 14 x + 24 = 9$$

Solve for x (complex solution)

$$x=\frac{2\sqrt{14}i}{7}-1\approx -1+1.069044968i$$

$$x=-\frac{2\sqrt{14}i}{7}-1\approx -1-1.069044968i$$

Steps Using the Quadratic Formula

All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.

$$7x^{2}+14x+24=9$$

Subtract 9 from both sides of the equation.

$$7x^{2}+14x+24-9=9-9$$

Subtracting 9 from itself leaves 0.

$$7x^{2}+14x+24-9=0$$

Subtract 9 from 24.

$$7x^{2}+14x+15=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 7 for a, 14 for b, and 15 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-14±\sqrt{14^{2}-4\times 7\times 15}}{2\times 7}$$

Square 14.

$$x=\frac{-14±\sqrt{196-4\times 7\times 15}}{2\times 7}$$

Multiply -4 times 7.

$$x=\frac{-14±\sqrt{196-28\times 15}}{2\times 7}$$

Multiply -28 times 15.

$$x=\frac{-14±\sqrt{196-420}}{2\times 7}$$

Add 196 to -420.

$$x=\frac{-14±\sqrt{-224}}{2\times 7}$$

Take the square root of -224.

$$x=\frac{-14±4\sqrt{14}i}{2\times 7}$$

Multiply 2 times 7.

$$x=\frac{-14±4\sqrt{14}i}{14}$$

Now solve the equation x=\frac{-14±4\sqrt{14}i}{14} when ± is plus. Add -14 to 4i\sqrt{14}.

$$x=\frac{-14+4\sqrt{14}i}{14}$$

Divide -14+4i\sqrt{14} by 14.

$$x=\frac{2\sqrt{14}i}{7}-1$$

Now solve the equation x=\frac{-14±4\sqrt{14}i}{14} when ± is minus. Subtract 4i\sqrt{14} from -14.

$$x=\frac{-4\sqrt{14}i-14}{14}$$

Divide -14-4i\sqrt{14} by 14.

$$x=-\frac{2\sqrt{14}i}{7}-1$$

The equation is now solved.

$$x=\frac{2\sqrt{14}i}{7}-1 x=-\frac{2\sqrt{14}i}{7}-1$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$7x^{2}+14x+24=9$$

Subtract 24 from both sides of the equation.

$$7x^{2}+14x+24-24=9-24$$

Subtracting 24 from itself leaves 0.

$$7x^{2}+14x=9-24$$

Subtract 24 from 9.

$$7x^{2}+14x=-15$$

Divide both sides by 7.

$$\frac{7x^{2}+14x}{7}=-\frac{15}{7}$$

Dividing by 7 undoes the multiplication by 7.

$$x^{2}+\frac{14}{7}x=-\frac{15}{7}$$

Divide 14 by 7.

$$x^{2}+2x=-\frac{15}{7}$$

Divide 2, the coefficient of the x term, by 2 to get 1. Then add the square of 1 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+2x+1^{2}=-\frac{15}{7}+1^{2}$$

Square 1.

$$x^{2}+2x+1=-\frac{15}{7}+1$$

Add -\frac{15}{7} to 1.

$$x^{2}+2x+1=-\frac{8}{7}$$

Factor x^{2}+2x+1. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+1\right)^{2}=-\frac{8}{7}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+1\right)^{2}}=\sqrt{-\frac{8}{7}}$$

Simplify.

$$x+1=\frac{2\sqrt{14}i}{7} x+1=-\frac{2\sqrt{14}i}{7}$$

Subtract 1 from both sides of the equation.

$$x=\frac{2\sqrt{14}i}{7}-1 x=-\frac{2\sqrt{14}i}{7}-1$$