Solve 21x^2+31x+4=0

$$21 { x }^{ 2 } +31x+4=0$$

Solve for x

$$x = -\frac{4}{3} = -1\frac{1}{3} \approx -1.333333333$$

$$x=-\frac{1}{7}\approx -0.142857143$$

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 21x^{2}+ax+bx+4. To find a and b, set up a system to be solved.

$$a+b=31 ab=21\times 4=84$$

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 84.

$$1,84 2,42 3,28 4,21 6,14 7,12$$

Calculate the sum for each pair.

$$1+84=85 2+42=44 3+28=31 4+21=25 6+14=20 7+12=19$$

The solution is the pair that gives sum 31.

$$a=3 b=28$$

Rewrite 21x^{2}+31x+4 as \left(21x^{2}+3x\right)+\left(28x+4\right).

$$\left(21x^{2}+3x\right)+\left(28x+4\right)$$

Factor out 3x in the first and 4 in the second group.

$$3x\left(7x+1\right)+4\left(7x+1\right)$$

Factor out common term 7x+1 by using distributive property.

$$\left(7x+1\right)\left(3x+4\right)$$

To find equation solutions, solve 7x+1=0 and 3x+4=0.

$$x=-\frac{1}{7} x=-\frac{4}{3}$$

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.

$$21x^{2}+31x+4=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 21 for a, 31 for b, and 4 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-31±\sqrt{31^{2}-4\times 21\times 4}}{2\times 21}$$

Square 31.

$$x=\frac{-31±\sqrt{961-4\times 21\times 4}}{2\times 21}$$

Multiply -4 times 21.

$$x=\frac{-31±\sqrt{961-84\times 4}}{2\times 21}$$

Multiply -84 times 4.

$$x=\frac{-31±\sqrt{961-336}}{2\times 21}$$

Add 961 to -336.

$$x=\frac{-31±\sqrt{625}}{2\times 21}$$

Take the square root of 625.

$$x=\frac{-31±25}{2\times 21}$$

Multiply 2 times 21.

$$x=\frac{-31±25}{42}$$

Now solve the equation x=\frac{-31±25}{42} when ± is plus. Add -31 to 25.

$$x=-\frac{6}{42}$$

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

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

Now solve the equation x=\frac{-31±25}{42} when ± is minus. Subtract 25 from -31.

$$x=-\frac{56}{42}$$

Reduce the fraction \frac{-56}{42} to lowest terms by extracting and canceling out 14.

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

The equation is now solved.

$$x=-\frac{1}{7} x=-\frac{4}{3}$$

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.

$$21x^{2}+31x+4=0$$

Subtract 4 from both sides of the equation.

$$21x^{2}+31x+4-4=-4$$

Subtracting 4 from itself leaves 0.

$$21x^{2}+31x=-4$$

Divide both sides by 21.

$$\frac{21x^{2}+31x}{21}=-\frac{4}{21}$$

Dividing by 21 undoes the multiplication by 21.

$$x^{2}+\frac{31}{21}x=-\frac{4}{21}$$

Divide \frac{31}{21}, the coefficient of the x term, by 2 to get \frac{31}{42}. Then add the square of \frac{31}{42} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{31}{21}x+\left(\frac{31}{42}\right)^{2}=-\frac{4}{21}+\left(\frac{31}{42}\right)^{2}$$

Square \frac{31}{42} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{31}{21}x+\frac{961}{1764}=-\frac{4}{21}+\frac{961}{1764}$$

Add -\frac{4}{21} to \frac{961}{1764} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{31}{21}x+\frac{961}{1764}=\frac{625}{1764}$$

Factor x^{2}+\frac{31}{21}x+\frac{961}{1764}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+\frac{31}{42}\right)^{2}=\frac{625}{1764}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{31}{42}\right)^{2}}=\sqrt{\frac{625}{1764}}$$

Simplify.

$$x+\frac{31}{42}=\frac{25}{42} x+\frac{31}{42}=-\frac{25}{42}$$

Subtract \frac{31}{42} from both sides of the equation.

$$x=-\frac{1}{7} x=-\frac{4}{3}$$