Solve 4x^2+5x+6=21

$$4 x ^ { 2 } + 5 x + 6 = 21$$

Solve for x

$$x = \frac{\sqrt{265} - 5}{8} \approx 1.409852575$$

$$x=\frac{-\sqrt{265}-5}{8}\approx -2.659852575$$

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.

$$4x^{2}+5x+6=21$$

Subtract 21 from both sides of the equation.

$$4x^{2}+5x+6-21=21-21$$

Subtracting 21 from itself leaves 0.

$$4x^{2}+5x+6-21=0$$

Subtract 21 from 6.

$$4x^{2}+5x-15=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 4 for a, 5 for b, and -15 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-5±\sqrt{5^{2}-4\times 4\left(-15\right)}}{2\times 4}$$

Square 5.

$$x=\frac{-5±\sqrt{25-4\times 4\left(-15\right)}}{2\times 4}$$

Multiply -4 times 4.

$$x=\frac{-5±\sqrt{25-16\left(-15\right)}}{2\times 4}$$

Multiply -16 times -15.

$$x=\frac{-5±\sqrt{25+240}}{2\times 4}$$

Add 25 to 240.

$$x=\frac{-5±\sqrt{265}}{2\times 4}$$

Multiply 2 times 4.

$$x=\frac{-5±\sqrt{265}}{8}$$

Now solve the equation x=\frac{-5±\sqrt{265}}{8} when ± is plus. Add -5 to \sqrt{265}.

$$x=\frac{\sqrt{265}-5}{8}$$

Now solve the equation x=\frac{-5±\sqrt{265}}{8} when ± is minus. Subtract \sqrt{265} from -5.

$$x=\frac{-\sqrt{265}-5}{8}$$

The equation is now solved.

$$x=\frac{\sqrt{265}-5}{8} x=\frac{-\sqrt{265}-5}{8}$$

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.

$$4x^{2}+5x+6=21$$

Subtract 6 from both sides of the equation.

$$4x^{2}+5x+6-6=21-6$$

Subtracting 6 from itself leaves 0.

$$4x^{2}+5x=21-6$$

Subtract 6 from 21.

$$4x^{2}+5x=15$$

Divide both sides by 4.

$$\frac{4x^{2}+5x}{4}=\frac{15}{4}$$

Dividing by 4 undoes the multiplication by 4.

$$x^{2}+\frac{5}{4}x=\frac{15}{4}$$

Divide \frac{5}{4}, the coefficient of the x term, by 2 to get \frac{5}{8}. Then add the square of \frac{5}{8} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{5}{4}x+\left(\frac{5}{8}\right)^{2}=\frac{15}{4}+\left(\frac{5}{8}\right)^{2}$$

Square \frac{5}{8} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{5}{4}x+\frac{25}{64}=\frac{15}{4}+\frac{25}{64}$$

Add \frac{15}{4} to \frac{25}{64} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{5}{4}x+\frac{25}{64}=\frac{265}{64}$$

Factor x^{2}+\frac{5}{4}x+\frac{25}{64}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+\frac{5}{8}\right)^{2}=\frac{265}{64}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{5}{8}\right)^{2}}=\sqrt{\frac{265}{64}}$$

Simplify.

$$x+\frac{5}{8}=\frac{\sqrt{265}}{8} x+\frac{5}{8}=-\frac{\sqrt{265}}{8}$$

Subtract \frac{5}{8} from both sides of the equation.

$$x=\frac{\sqrt{265}-5}{8} x=\frac{-\sqrt{265}-5}{8}$$