Solve 3x^2-18x+11=6

$$3 x ^ { 2 } - 18 x + 11 = 6$$

Solve for x

$$x=\frac{\sqrt{66}}{3}+3\approx 5.708012802$$

$$x=-\frac{\sqrt{66}}{3}+3\approx 0.291987198$$

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.

$$3x^{2}-18x+11=6$$

Subtract 6 from both sides of the equation.

$$3x^{2}-18x+11-6=6-6$$

Subtracting 6 from itself leaves 0.

$$3x^{2}-18x+11-6=0$$

Subtract 6 from 11.

$$3x^{2}-18x+5=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 3 for a, -18 for b, and 5 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-18\right)±\sqrt{\left(-18\right)^{2}-4\times 3\times 5}}{2\times 3}$$

Square -18.

$$x=\frac{-\left(-18\right)±\sqrt{324-4\times 3\times 5}}{2\times 3}$$

Multiply -4 times 3.

$$x=\frac{-\left(-18\right)±\sqrt{324-12\times 5}}{2\times 3}$$

Multiply -12 times 5.

$$x=\frac{-\left(-18\right)±\sqrt{324-60}}{2\times 3}$$

Add 324 to -60.

$$x=\frac{-\left(-18\right)±\sqrt{264}}{2\times 3}$$

Take the square root of 264.

$$x=\frac{-\left(-18\right)±2\sqrt{66}}{2\times 3}$$

The opposite of -18 is 18.

$$x=\frac{18±2\sqrt{66}}{2\times 3}$$

Multiply 2 times 3.

$$x=\frac{18±2\sqrt{66}}{6}$$

Now solve the equation x=\frac{18±2\sqrt{66}}{6} when ± is plus. Add 18 to 2\sqrt{66}.

$$x=\frac{2\sqrt{66}+18}{6}$$

Divide 18+2\sqrt{66} by 6.

$$x=\frac{\sqrt{66}}{3}+3$$

Now solve the equation x=\frac{18±2\sqrt{66}}{6} when ± is minus. Subtract 2\sqrt{66} from 18.

$$x=\frac{18-2\sqrt{66}}{6}$$

Divide 18-2\sqrt{66} by 6.

$$x=-\frac{\sqrt{66}}{3}+3$$

The equation is now solved.

$$x=\frac{\sqrt{66}}{3}+3 x=-\frac{\sqrt{66}}{3}+3$$

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.

$$3x^{2}-18x+11=6$$

Subtract 11 from both sides of the equation.

$$3x^{2}-18x+11-11=6-11$$

Subtracting 11 from itself leaves 0.

$$3x^{2}-18x=6-11$$

Subtract 11 from 6.

$$3x^{2}-18x=-5$$

Divide both sides by 3.

$$\frac{3x^{2}-18x}{3}=-\frac{5}{3}$$

Dividing by 3 undoes the multiplication by 3.

$$x^{2}+\left(-\frac{18}{3}\right)x=-\frac{5}{3}$$

Divide -18 by 3.

$$x^{2}-6x=-\frac{5}{3}$$

Divide -6, the coefficient of the x term, by 2 to get -3. Then add the square of -3 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-6x+\left(-3\right)^{2}=-\frac{5}{3}+\left(-3\right)^{2}$$

Square -3.

$$x^{2}-6x+9=-\frac{5}{3}+9$$

Add -\frac{5}{3} to 9.

$$x^{2}-6x+9=\frac{22}{3}$$

Factor x^{2}-6x+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}.

$$\left(x-3\right)^{2}=\frac{22}{3}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-3\right)^{2}}=\sqrt{\frac{22}{3}}$$

Simplify.

$$x-3=\frac{\sqrt{66}}{3} x-3=-\frac{\sqrt{66}}{3}$$

Add 3 to both sides of the equation.

$$x=\frac{\sqrt{66}}{3}+3 x=-\frac{\sqrt{66}}{3}+3$$