Solve -2x^2+40x-72=0

$$-2 { x }^{ 2 } +40x-72=0$$

Solve for x

$$x=2$$

$$x=18$$

Steps Using Factoring By Grouping

Divide both sides by 2.

$$-x^{2}+20x-36=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -x^{2}+ax+bx-36. To find a and b, set up a system to be solved.

$$a+b=20 ab=-\left(-36\right)=36$$

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 36.

$$1,36 2,18 3,12 4,9 6,6$$

Calculate the sum for each pair.

$$1+36=37 2+18=20 3+12=15 4+9=13 6+6=12$$

The solution is the pair that gives sum 20.

$$a=18 b=2$$

Rewrite -x^{2}+20x-36 as \left(-x^{2}+18x\right)+\left(2x-36\right).

$$\left(-x^{2}+18x\right)+\left(2x-36\right)$$

Factor out -x in the first and 2 in the second group.

$$-x\left(x-18\right)+2\left(x-18\right)$$

Factor out common term x-18 by using distributive property.

$$\left(x-18\right)\left(-x+2\right)$$

To find equation solutions, solve x-18=0 and -x+2=0.

$$x=18 x=2$$

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.

$$-2x^{2}+40x-72=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -2 for a, 40 for b, and -72 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-40±\sqrt{40^{2}-4\left(-2\right)\left(-72\right)}}{2\left(-2\right)}$$

Square 40.

$$x=\frac{-40±\sqrt{1600-4\left(-2\right)\left(-72\right)}}{2\left(-2\right)}$$

Multiply -4 times -2.

$$x=\frac{-40±\sqrt{1600+8\left(-72\right)}}{2\left(-2\right)}$$

Multiply 8 times -72.

$$x=\frac{-40±\sqrt{1600-576}}{2\left(-2\right)}$$

Add 1600 to -576.

$$x=\frac{-40±\sqrt{1024}}{2\left(-2\right)}$$

Take the square root of 1024.

$$x=\frac{-40±32}{2\left(-2\right)}$$

Multiply 2 times -2.

$$x=\frac{-40±32}{-4}$$

Now solve the equation x=\frac{-40±32}{-4} when ± is plus. Add -40 to 32.

$$x=-\frac{8}{-4}$$

Divide -8 by -4.

$$x=2$$

Now solve the equation x=\frac{-40±32}{-4} when ± is minus. Subtract 32 from -40.

$$x=-\frac{72}{-4}$$

Divide -72 by -4.

$$x=18$$

The equation is now solved.

$$x=2 x=18$$

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.

$$-2x^{2}+40x-72=0$$

Add 72 to both sides of the equation.

$$-2x^{2}+40x-72-\left(-72\right)=-\left(-72\right)$$

Subtracting -72 from itself leaves 0.

$$-2x^{2}+40x=-\left(-72\right)$$

Subtract -72 from 0.

$$-2x^{2}+40x=72$$

Divide both sides by -2.

$$\frac{-2x^{2}+40x}{-2}=\frac{72}{-2}$$

Dividing by -2 undoes the multiplication by -2.

$$x^{2}+\frac{40}{-2}x=\frac{72}{-2}$$

Divide 40 by -2.

$$x^{2}-20x=\frac{72}{-2}$$

Divide 72 by -2.

$$x^{2}-20x=-36$$

Divide -20, the coefficient of the x term, by 2 to get -10. Then add the square of -10 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-20x+\left(-10\right)^{2}=-36+\left(-10\right)^{2}$$

Square -10.

$$x^{2}-20x+100=-36+100$$

Add -36 to 100.

$$x^{2}-20x+100=64$$

Factor x^{2}-20x+100. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-10\right)^{2}=64$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-10\right)^{2}}=\sqrt{64}$$

Simplify.

$$x-10=8 x-10=-8$$

Add 10 to both sides of the equation.

$$x=18 x=2$$