Solve 9x^2+18x+9=16

$$9 x ^ { 2 } + 18 x + 9 = 16$$

Solve for x

$$x = -\frac{7}{3} = -2\frac{1}{3} \approx -2.333333333$$

$$x=\frac{1}{3}\approx 0.333333333$$

Steps Using Factoring By Grouping

Subtract 16 from both sides.

$$9x^{2}+18x+9-16=0$$

Subtract 16 from 9 to get -7.

$$9x^{2}+18x-7=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as 9x^{2}+ax+bx-7. To find a and b, set up a system to be solved.

$$a+b=18 ab=9\left(-7\right)=-63$$

Since ab is negative, a and b have the opposite signs. Since a+b is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product -63.

$$-1,63 -3,21 -7,9$$

Calculate the sum for each pair.

$$-1+63=62 -3+21=18 -7+9=2$$

The solution is the pair that gives sum 18.

$$a=-3 b=21$$

Rewrite 9x^{2}+18x-7 as \left(9x^{2}-3x\right)+\left(21x-7\right).

$$\left(9x^{2}-3x\right)+\left(21x-7\right)$$

Factor out 3x in the first and 7 in the second group.

$$3x\left(3x-1\right)+7\left(3x-1\right)$$

Factor out common term 3x-1 by using distributive property.

$$\left(3x-1\right)\left(3x+7\right)$$

To find equation solutions, solve 3x-1=0 and 3x+7=0.

$$x=\frac{1}{3} x=-\frac{7}{3}$$

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.

$$9x^{2}+18x+9=16$$

Subtract 16 from both sides of the equation.

$$9x^{2}+18x+9-16=16-16$$

Subtracting 16 from itself leaves 0.

$$9x^{2}+18x+9-16=0$$

Subtract 16 from 9.

$$9x^{2}+18x-7=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 9 for a, 18 for b, and -7 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-18±\sqrt{18^{2}-4\times 9\left(-7\right)}}{2\times 9}$$

Square 18.

$$x=\frac{-18±\sqrt{324-4\times 9\left(-7\right)}}{2\times 9}$$

Multiply -4 times 9.

$$x=\frac{-18±\sqrt{324-36\left(-7\right)}}{2\times 9}$$

Multiply -36 times -7.

$$x=\frac{-18±\sqrt{324+252}}{2\times 9}$$

Add 324 to 252.

$$x=\frac{-18±\sqrt{576}}{2\times 9}$$

Take the square root of 576.

$$x=\frac{-18±24}{2\times 9}$$

Multiply 2 times 9.

$$x=\frac{-18±24}{18}$$

Now solve the equation x=\frac{-18±24}{18} when ± is plus. Add -18 to 24.

$$x=\frac{6}{18}$$

Reduce the fraction \frac{6}{18} to lowest terms by extracting and canceling out 6.

$$x=\frac{1}{3}$$

Now solve the equation x=\frac{-18±24}{18} when ± is minus. Subtract 24 from -18.

$$x=-\frac{42}{18}$$

Reduce the fraction \frac{-42}{18} to lowest terms by extracting and canceling out 6.

$$x=-\frac{7}{3}$$

The equation is now solved.

$$x=\frac{1}{3} x=-\frac{7}{3}$$

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.

$$9x^{2}+18x+9=16$$

Subtract 9 from both sides of the equation.

$$9x^{2}+18x+9-9=16-9$$

Subtracting 9 from itself leaves 0.

$$9x^{2}+18x=16-9$$

Subtract 9 from 16.

$$9x^{2}+18x=7$$

Divide both sides by 9.

$$\frac{9x^{2}+18x}{9}=\frac{7}{9}$$

Dividing by 9 undoes the multiplication by 9.

$$x^{2}+\frac{18}{9}x=\frac{7}{9}$$

Divide 18 by 9.

$$x^{2}+2x=\frac{7}{9}$$

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.

$$x^{2}+2x+1^{2}=\frac{7}{9}+1^{2}$$

Square 1.

$$x^{2}+2x+1=\frac{7}{9}+1$$

Add \frac{7}{9} to 1.

$$x^{2}+2x+1=\frac{16}{9}$$

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}.

$$\left(x+1\right)^{2}=\frac{16}{9}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+1\right)^{2}}=\sqrt{\frac{16}{9}}$$

Simplify.

$$x+1=\frac{4}{3} x+1=-\frac{4}{3}$$

Subtract 1 from both sides of the equation.

$$x=\frac{1}{3} x=-\frac{7}{3}$$