Solve (x-2)(x+1)=4

$$(x-2)(x+1)=4$$

Solve for x

$$x=-2$$

$$x=3$$

Steps Using the Quadratic Formula

Use the distributive property to multiply x-2 by x+1 and combine like terms.

$$x^{2}-x-2=4$$

Subtract 4 from both sides.

$$x^{2}-x-2-4=0$$

Subtract 4 from -2 to get -6.

$$x^{2}-x-6=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -1 for b, and -6 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-1\right)±\sqrt{1-4\left(-6\right)}}{2}$$

Multiply -4 times -6.

$$x=\frac{-\left(-1\right)±\sqrt{1+24}}{2}$$

Add 1 to 24.

$$x=\frac{-\left(-1\right)±\sqrt{25}}{2}$$

Take the square root of 25.

$$x=\frac{-\left(-1\right)±5}{2}$$

The opposite of -1 is 1.

$$x=\frac{1±5}{2}$$

Now solve the equation x=\frac{1±5}{2} when ± is plus. Add 1 to 5.

$$x=\frac{6}{2}$$

Divide 6 by 2.

$$x=3$$

Now solve the equation x=\frac{1±5}{2} when ± is minus. Subtract 5 from 1.

$$x=-\frac{4}{2}$$

Divide -4 by 2.

$$x=-2$$

The equation is now solved.

$$x=3 x=-2$$

Steps for Completing the Square

Use the distributive property to multiply x-2 by x+1 and combine like terms.

$$x^{2}-x-2=4$$

Add 2 to both sides.

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

Add 4 and 2 to get 6.

$$x^{2}-x=6$$

Divide -1, the coefficient of the x term, by 2 to get -\frac{1}{2}. Then add the square of -\frac{1}{2} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-x+\left(-\frac{1}{2}\right)^{2}=6+\left(-\frac{1}{2}\right)^{2}$$

Square -\frac{1}{2} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-x+\frac{1}{4}=6+\frac{1}{4}$$

Add 6 to \frac{1}{4}.

$$x^{2}-x+\frac{1}{4}=\frac{25}{4}$$

Factor x^{2}-x+\frac{1}{4}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-\frac{1}{2}\right)^{2}=\frac{25}{4}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{1}{2}\right)^{2}}=\sqrt{\frac{25}{4}}$$

Simplify.

$$x-\frac{1}{2}=\frac{5}{2} x-\frac{1}{2}=-\frac{5}{2}$$

Add \frac{1}{2} to both sides of the equation.

$$x=3 x=-2$$