Solve 3x^2-8x-15=0

$$3 { x }^{ 2 } -8x-15 = 0$$

Solve for x

$$x = \frac{\sqrt{61} + 4}{3} \approx 3.936749892$$

$$x=\frac{4-\sqrt{61}}{3}\approx -1.270083225$$

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.

$$3x^{2}-8x-15=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 3 for a, -8 for b, and -15 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-8\right)±\sqrt{\left(-8\right)^{2}-4\times 3\left(-15\right)}}{2\times 3}$$

Square -8.

$$x=\frac{-\left(-8\right)±\sqrt{64-4\times 3\left(-15\right)}}{2\times 3}$$

Multiply -4 times 3.

$$x=\frac{-\left(-8\right)±\sqrt{64-12\left(-15\right)}}{2\times 3}$$

Multiply -12 times -15.

$$x=\frac{-\left(-8\right)±\sqrt{64+180}}{2\times 3}$$

Add 64 to 180.

$$x=\frac{-\left(-8\right)±\sqrt{244}}{2\times 3}$$

Take the square root of 244.

$$x=\frac{-\left(-8\right)±2\sqrt{61}}{2\times 3}$$

The opposite of -8 is 8.

$$x=\frac{8±2\sqrt{61}}{2\times 3}$$

Multiply 2 times 3.

$$x=\frac{8±2\sqrt{61}}{6}$$

Now solve the equation x=\frac{8±2\sqrt{61}}{6} when ± is plus. Add 8 to 2\sqrt{61}.

$$x=\frac{2\sqrt{61}+8}{6}$$

Divide 8+2\sqrt{61} by 6.

$$x=\frac{\sqrt{61}+4}{3}$$

Now solve the equation x=\frac{8±2\sqrt{61}}{6} when ± is minus. Subtract 2\sqrt{61} from 8.

$$x=\frac{8-2\sqrt{61}}{6}$$

Divide 8-2\sqrt{61} by 6.

$$x=\frac{4-\sqrt{61}}{3}$$

The equation is now solved.

$$x=\frac{\sqrt{61}+4}{3} x=\frac{4-\sqrt{61}}{3}$$

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.

$$3x^{2}-8x-15=0$$

Add 15 to both sides of the equation.

$$3x^{2}-8x-15-\left(-15\right)=-\left(-15\right)$$

Subtracting -15 from itself leaves 0.

$$3x^{2}-8x=-\left(-15\right)$$

Subtract -15 from 0.

$$3x^{2}-8x=15$$

Divide both sides by 3.

$$\frac{3x^{2}-8x}{3}=\frac{15}{3}$$

Dividing by 3 undoes the multiplication by 3.

$$x^{2}-\frac{8}{3}x=\frac{15}{3}$$

Divide 15 by 3.

$$x^{2}-\frac{8}{3}x=5$$

Divide -\frac{8}{3}, the coefficient of the x term, by 2 to get -\frac{4}{3}. Then add the square of -\frac{4}{3} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-\frac{8}{3}x+\left(-\frac{4}{3}\right)^{2}=5+\left(-\frac{4}{3}\right)^{2}$$

Square -\frac{4}{3} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-\frac{8}{3}x+\frac{16}{9}=5+\frac{16}{9}$$

Add 5 to \frac{16}{9}.

$$x^{2}-\frac{8}{3}x+\frac{16}{9}=\frac{61}{9}$$

Factor x^{2}-\frac{8}{3}x+\frac{16}{9}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-\frac{4}{3}\right)^{2}=\frac{61}{9}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{4}{3}\right)^{2}}=\sqrt{\frac{61}{9}}$$

Simplify.

$$x-\frac{4}{3}=\frac{\sqrt{61}}{3} x-\frac{4}{3}=-\frac{\sqrt{61}}{3}$$

Add \frac{4}{3} to both sides of the equation.

$$x=\frac{\sqrt{61}+4}{3} x=\frac{4-\sqrt{61}}{3}$$