Solve (x-1)(x-2)=18

$$( x - 1 ) ( x - 2 ) = 18$$

Solve for x

$$x = \frac{\sqrt{73} + 3}{2} \approx 5.772001873$$

$$x=\frac{3-\sqrt{73}}{2}\approx -2.772001873$$

Steps Using the Quadratic Formula

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

$$x^{2}-3x+2=18$$

Subtract 18 from both sides.

$$x^{2}-3x+2-18=0$$

Subtract 18 from 2 to get -16.

$$x^{2}-3x-16=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -3 for b, and -16 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-3\right)±\sqrt{\left(-3\right)^{2}-4\left(-16\right)}}{2}$$

Square -3.

$$x=\frac{-\left(-3\right)±\sqrt{9-4\left(-16\right)}}{2}$$

Multiply -4 times -16.

$$x=\frac{-\left(-3\right)±\sqrt{9+64}}{2}$$

Add 9 to 64.

$$x=\frac{-\left(-3\right)±\sqrt{73}}{2}$$

The opposite of -3 is 3.

$$x=\frac{3±\sqrt{73}}{2}$$

Now solve the equation x=\frac{3±\sqrt{73}}{2} when ± is plus. Add 3 to \sqrt{73}.

$$x=\frac{\sqrt{73}+3}{2}$$

Now solve the equation x=\frac{3±\sqrt{73}}{2} when ± is minus. Subtract \sqrt{73} from 3.

$$x=\frac{3-\sqrt{73}}{2}$$

The equation is now solved.

$$x=\frac{\sqrt{73}+3}{2} x=\frac{3-\sqrt{73}}{2}$$

Steps for Completing the Square

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

$$x^{2}-3x+2=18$$

Subtract 2 from both sides.

$$x^{2}-3x=18-2$$

Subtract 2 from 18 to get 16.

$$x^{2}-3x=16$$

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

$$x^{2}-3x+\left(-\frac{3}{2}\right)^{2}=16+\left(-\frac{3}{2}\right)^{2}$$

Square -\frac{3}{2} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-3x+\frac{9}{4}=16+\frac{9}{4}$$

Add 16 to \frac{9}{4}.

$$x^{2}-3x+\frac{9}{4}=\frac{73}{4}$$

Factor x^{2}-3x+\frac{9}{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}.

$$\left(x-\frac{3}{2}\right)^{2}=\frac{73}{4}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{3}{2}\right)^{2}}=\sqrt{\frac{73}{4}}$$

Simplify.

$$x-\frac{3}{2}=\frac{\sqrt{73}}{2} x-\frac{3}{2}=-\frac{\sqrt{73}}{2}$$

Add \frac{3}{2} to both sides of the equation.

$$x=\frac{\sqrt{73}+3}{2} x=\frac{3-\sqrt{73}}{2}$$