Solve 2x^2+28x=-66

$$2 { x }^{ 2 } +28x=-66$$

Solve for x

$$x=-11$$

$$x=-3$$

Steps Using Factoring By Grouping

Add 66 to both sides.

$$2x^{2}+28x+66=0$$

Divide both sides by 2.

$$x^{2}+14x+33=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as x^{2}+ax+bx+33. To find a and b, set up a system to be solved.

$$a+b=14 ab=1\times 33=33$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. List all such integer pairs that give product 33.

$$1,33 3,11$$

Calculate the sum for each pair.

$$1+33=34 3+11=14$$

The solution is the pair that gives sum 14.

$$a=3 b=11$$

Rewrite x^{2}+14x+33 as \left(x^{2}+3x\right)+\left(11x+33\right).

$$\left(x^{2}+3x\right)+\left(11x+33\right)$$

Factor out x in the first and 11 in the second group.

$$x\left(x+3\right)+11\left(x+3\right)$$

Factor out common term x+3 by using distributive property.

$$\left(x+3\right)\left(x+11\right)$$

To find equation solutions, solve x+3=0 and x+11=0.

$$x=-3 x=-11$$

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.

$$2x^{2}+28x=-66$$

Add 66 to both sides of the equation.

$$2x^{2}+28x-\left(-66\right)=-66-\left(-66\right)$$

Subtracting -66 from itself leaves 0.

$$2x^{2}+28x-\left(-66\right)=0$$

Subtract -66 from 0.

$$2x^{2}+28x+66=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 2 for a, 28 for b, and 66 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-28±\sqrt{28^{2}-4\times 2\times 66}}{2\times 2}$$

Square 28.

$$x=\frac{-28±\sqrt{784-4\times 2\times 66}}{2\times 2}$$

Multiply -4 times 2.

$$x=\frac{-28±\sqrt{784-8\times 66}}{2\times 2}$$

Multiply -8 times 66.

$$x=\frac{-28±\sqrt{784-528}}{2\times 2}$$

Add 784 to -528.

$$x=\frac{-28±\sqrt{256}}{2\times 2}$$

Take the square root of 256.

$$x=\frac{-28±16}{2\times 2}$$

Multiply 2 times 2.

$$x=\frac{-28±16}{4}$$

Now solve the equation x=\frac{-28±16}{4} when ± is plus. Add -28 to 16.

$$x=-\frac{12}{4}$$

Divide -12 by 4.

$$x=-3$$

Now solve the equation x=\frac{-28±16}{4} when ± is minus. Subtract 16 from -28.

$$x=-\frac{44}{4}$$

Divide -44 by 4.

$$x=-11$$

The equation is now solved.

$$x=-3 x=-11$$

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.

$$2x^{2}+28x=-66$$

Divide both sides by 2.

$$\frac{2x^{2}+28x}{2}=-\frac{66}{2}$$

Dividing by 2 undoes the multiplication by 2.

$$x^{2}+\frac{28}{2}x=-\frac{66}{2}$$

Divide 28 by 2.

$$x^{2}+14x=-\frac{66}{2}$$

Divide -66 by 2.

$$x^{2}+14x=-33$$

Divide 14, the coefficient of the x term, by 2 to get 7. Then add the square of 7 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+14x+7^{2}=-33+7^{2}$$

Square 7.

$$x^{2}+14x+49=-33+49$$

Add -33 to 49.

$$x^{2}+14x+49=16$$

Factor x^{2}+14x+49. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+7\right)^{2}=16$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+7\right)^{2}}=\sqrt{16}$$

Simplify.

$$x+7=4 x+7=-4$$

Subtract 7 from both sides of the equation.

$$x=-3 x=-11$$