Solve left(40-xright)left(20+2xright)=1200

$$\left( 40-x \right) \left( 20+2x \right) = 1200$$

Solve for x

$$x=10$$

$$x=20$$

Steps Using the Quadratic Formula

Use the distributive property to multiply 40-x by 20+2x and combine like terms.

$$800+60x-2x^{2}=1200$$

Subtract 1200 from both sides.

$$800+60x-2x^{2}-1200=0$$

Subtract 1200 from 800 to get -400.

$$-400+60x-2x^{2}=0$$

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.

$$-2x^{2}+60x-400=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -2 for a, 60 for b, and -400 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-60±\sqrt{60^{2}-4\left(-2\right)\left(-400\right)}}{2\left(-2\right)}$$

Square 60.

$$x=\frac{-60±\sqrt{3600-4\left(-2\right)\left(-400\right)}}{2\left(-2\right)}$$

Multiply -4 times -2.

$$x=\frac{-60±\sqrt{3600+8\left(-400\right)}}{2\left(-2\right)}$$

Multiply 8 times -400.

$$x=\frac{-60±\sqrt{3600-3200}}{2\left(-2\right)}$$

Add 3600 to -3200.

$$x=\frac{-60±\sqrt{400}}{2\left(-2\right)}$$

Take the square root of 400.

$$x=\frac{-60±20}{2\left(-2\right)}$$

Multiply 2 times -2.

$$x=\frac{-60±20}{-4}$$

Now solve the equation x=\frac{-60±20}{-4} when ± is plus. Add -60 to 20.

$$x=-\frac{40}{-4}$$

Divide -40 by -4.

$$x=10$$

Now solve the equation x=\frac{-60±20}{-4} when ± is minus. Subtract 20 from -60.

$$x=-\frac{80}{-4}$$

Divide -80 by -4.

$$x=20$$

The equation is now solved.

$$x=10 x=20$$

Steps for Completing the Square

Use the distributive property to multiply 40-x by 20+2x and combine like terms.

$$800+60x-2x^{2}=1200$$

Subtract 800 from both sides.

$$60x-2x^{2}=1200-800$$

Subtract 800 from 1200 to get 400.

$$60x-2x^{2}=400$$

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.

$$-2x^{2}+60x=400$$

Divide both sides by -2.

$$\frac{-2x^{2}+60x}{-2}=\frac{400}{-2}$$

Dividing by -2 undoes the multiplication by -2.

$$x^{2}+\frac{60}{-2}x=\frac{400}{-2}$$

Divide 60 by -2.

$$x^{2}-30x=\frac{400}{-2}$$

Divide 400 by -2.

$$x^{2}-30x=-200$$

Divide -30, the coefficient of the x term, by 2 to get -15. Then add the square of -15 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-30x+\left(-15\right)^{2}=-200+\left(-15\right)^{2}$$

Square -15.

$$x^{2}-30x+225=-200+225$$

Add -200 to 225.

$$x^{2}-30x+225=25$$

Factor x^{2}-30x+225. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-15\right)^{2}=25$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-15\right)^{2}}=\sqrt{25}$$

Simplify.

$$x-15=5 x-15=-5$$

Add 15 to both sides of the equation.

$$x=20 x=10$$