Solve 9x^2+42x+49=0

$$9 { x }^{ 2 } +42x+49=0$$

Solve for x

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

Steps Using Factoring By Grouping

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as 9x^{2}+ax+bx+49. To find a and b, set up a system to be solved.

$$a+b=42 ab=9\times 49=441$$

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 441.

$$1,441 3,147 7,63 9,49 21,21$$

Calculate the sum for each pair.

$$1+441=442 3+147=150 7+63=70 9+49=58 21+21=42$$

The solution is the pair that gives sum 42.

$$a=21 b=21$$

Rewrite 9x^{2}+42x+49 as \left(9x^{2}+21x\right)+\left(21x+49\right).

$$\left(9x^{2}+21x\right)+\left(21x+49\right)$$

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

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

Factor out common term 3x+7 by using distributive property.

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

Rewrite as a binomial square.

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

To find equation solution, solve 3x+7=0.

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

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.

$$9x^{2}+42x+49=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 9 for a, 42 for b, and 49 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-42±\sqrt{42^{2}-4\times 9\times 49}}{2\times 9}$$

Square 42.

$$x=\frac{-42±\sqrt{1764-4\times 9\times 49}}{2\times 9}$$

Multiply -4 times 9.

$$x=\frac{-42±\sqrt{1764-36\times 49}}{2\times 9}$$

Multiply -36 times 49.

$$x=\frac{-42±\sqrt{1764-1764}}{2\times 9}$$

Add 1764 to -1764.

$$x=\frac{-42±\sqrt{0}}{2\times 9}$$

Take the square root of 0.

$$x=-\frac{42}{2\times 9}$$

Multiply 2 times 9.

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

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

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

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.

$$9x^{2}+42x+49=0$$

Subtract 49 from both sides of the equation.

$$9x^{2}+42x+49-49=-49$$

Subtracting 49 from itself leaves 0.

$$9x^{2}+42x=-49$$

Divide both sides by 9.

$$\frac{9x^{2}+42x}{9}=-\frac{49}{9}$$

Dividing by 9 undoes the multiplication by 9.

$$x^{2}+\frac{42}{9}x=-\frac{49}{9}$$

Reduce the fraction \frac{42}{9} to lowest terms by extracting and canceling out 3.

$$x^{2}+\frac{14}{3}x=-\frac{49}{9}$$

Divide \frac{14}{3}, the coefficient of the x term, by 2 to get \frac{7}{3}. Then add the square of \frac{7}{3} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{14}{3}x+\left(\frac{7}{3}\right)^{2}=-\frac{49}{9}+\left(\frac{7}{3}\right)^{2}$$

Square \frac{7}{3} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{14}{3}x+\frac{49}{9}=\frac{-49+49}{9}$$

Add -\frac{49}{9} to \frac{49}{9} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{14}{3}x+\frac{49}{9}=0$$

Factor x^{2}+\frac{14}{3}x+\frac{49}{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}.

$$\left(x+\frac{7}{3}\right)^{2}=0$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{7}{3}\right)^{2}}=\sqrt{0}$$

Simplify.

$$x+\frac{7}{3}=0 x+\frac{7}{3}=0$$

Subtract \frac{7}{3} from both sides of the equation.

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

The equation is now solved. Solutions are the same.

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