Solve -x^2+32x-236=20

$$- { x }^{ 2 } +32x-236=20$$

Solve for x

$$x=16$$

Steps Using Factoring By Grouping

Subtract 20 from both sides.

$$-x^{2}+32x-236-20=0$$

Subtract 20 from -236 to get -256.

$$-x^{2}+32x-256=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -x^{2}+ax+bx-256. To find a and b, set up a system to be solved.

$$a+b=32 ab=-\left(-256\right)=256$$

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 256.

$$1,256 2,128 4,64 8,32 16,16$$

Calculate the sum for each pair.

$$1+256=257 2+128=130 4+64=68 8+32=40 16+16=32$$

The solution is the pair that gives sum 32.

$$a=16 b=16$$

Rewrite -x^{2}+32x-256 as \left(-x^{2}+16x\right)+\left(16x-256\right).

$$\left(-x^{2}+16x\right)+\left(16x-256\right)$$

Factor out -x in the first and 16 in the second group.

$$-x\left(x-16\right)+16\left(x-16\right)$$

Factor out common term x-16 by using distributive property.

$$\left(x-16\right)\left(-x+16\right)$$

To find equation solutions, solve x-16=0 and -x+16=0.

$$x=16 x=16$$

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.

$$-x^{2}+32x-236=20$$

Subtract 20 from both sides of the equation.

$$-x^{2}+32x-236-20=20-20$$

Subtracting 20 from itself leaves 0.

$$-x^{2}+32x-236-20=0$$

Subtract 20 from -236.

$$-x^{2}+32x-256=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 32 for b, and -256 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-32±\sqrt{32^{2}-4\left(-1\right)\left(-256\right)}}{2\left(-1\right)}$$

Square 32.

$$x=\frac{-32±\sqrt{1024-4\left(-1\right)\left(-256\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-32±\sqrt{1024+4\left(-256\right)}}{2\left(-1\right)}$$

Multiply 4 times -256.

$$x=\frac{-32±\sqrt{1024-1024}}{2\left(-1\right)}$$

Add 1024 to -1024.

$$x=\frac{-32±\sqrt{0}}{2\left(-1\right)}$$

Take the square root of 0.

$$x=-\frac{32}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=-\frac{32}{-2}$$

Divide -32 by -2.

$$x=16$$

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.

$$-x^{2}+32x-236=20$$

Add 236 to both sides of the equation.

$$-x^{2}+32x-236-\left(-236\right)=20-\left(-236\right)$$

Subtracting -236 from itself leaves 0.

$$-x^{2}+32x=20-\left(-236\right)$$

Subtract -236 from 20.

$$-x^{2}+32x=256$$

Divide both sides by -1.

$$\frac{-x^{2}+32x}{-1}=\frac{256}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\frac{32}{-1}x=\frac{256}{-1}$$

Divide 32 by -1.

$$x^{2}-32x=\frac{256}{-1}$$

Divide 256 by -1.

$$x^{2}-32x=-256$$

Divide -32, the coefficient of the x term, by 2 to get -16. Then add the square of -16 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-32x+\left(-16\right)^{2}=-256+\left(-16\right)^{2}$$

Square -16.

$$x^{2}-32x+256=-256+256$$

Add -256 to 256.

$$x^{2}-32x+256=0$$

Factor x^{2}-32x+256. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-16\right)^{2}=0$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-16\right)^{2}}=\sqrt{0}$$

Simplify.

$$x-16=0 x-16=0$$

Add 16 to both sides of the equation.

$$x=16 x=16$$

The equation is now solved. Solutions are the same.

$$x=16$$