Solve 40x-x^2=400

$$40 x - x ^ { 2 } = 400$$

Solve for x

$$x=20$$

Steps Using Factoring By Grouping

Subtract 400 from both sides.

$$40x-x^{2}-400=0$$

Rearrange the polynomial to put it in standard form. Place the terms in order from highest to lowest power.

$$-x^{2}+40x-400=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -x^{2}+ax+bx-400. To find a and b, set up a system to be solved.

$$a+b=40 ab=-\left(-400\right)=400$$

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 400.

$$1,400 2,200 4,100 5,80 8,50 10,40 16,25 20,20$$

Calculate the sum for each pair.

$$1+400=401 2+200=202 4+100=104 5+80=85 8+50=58 10+40=50 16+25=41 20+20=40$$

The solution is the pair that gives sum 40.

$$a=20 b=20$$

Rewrite -x^{2}+40x-400 as \left(-x^{2}+20x\right)+\left(20x-400\right).

$$\left(-x^{2}+20x\right)+\left(20x-400\right)$$

Factor out -x in the first and 20 in the second group.

$$-x\left(x-20\right)+20\left(x-20\right)$$

Factor out common term x-20 by using distributive property.

$$\left(x-20\right)\left(-x+20\right)$$

To find equation solutions, solve x-20=0 and -x+20=0.

$$x=20 x=20$$

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.

$$-x^{2}+40x=400$$

Subtract 400 from both sides of the equation.

$$-x^{2}+40x-400=400-400$$

Subtracting 400 from itself leaves 0.

$$-x^{2}+40x-400=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 40 for b, and -400 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-40±\sqrt{40^{2}-4\left(-1\right)\left(-400\right)}}{2\left(-1\right)}$$

Square 40.

$$x=\frac{-40±\sqrt{1600-4\left(-1\right)\left(-400\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-40±\sqrt{1600+4\left(-400\right)}}{2\left(-1\right)}$$

Multiply 4 times -400.

$$x=\frac{-40±\sqrt{1600-1600}}{2\left(-1\right)}$$

Add 1600 to -1600.

$$x=\frac{-40±\sqrt{0}}{2\left(-1\right)}$$

Take the square root of 0.

$$x=-\frac{40}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=-\frac{40}{-2}$$

Divide -40 by -2.

$$x=20$$

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.

$$-x^{2}+40x=400$$

Divide both sides by -1.

$$\frac{-x^{2}+40x}{-1}=\frac{400}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\frac{40}{-1}x=\frac{400}{-1}$$

Divide 40 by -1.

$$x^{2}-40x=\frac{400}{-1}$$

Divide 400 by -1.

$$x^{2}-40x=-400$$

Divide -40, the coefficient of the x term, by 2 to get -20. Then add the square of -20 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-40x+\left(-20\right)^{2}=-400+\left(-20\right)^{2}$$

Square -20.

$$x^{2}-40x+400=-400+400$$

Add -400 to 400.

$$x^{2}-40x+400=0$$

Factor x^{2}-40x+400. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-20\right)^{2}=0$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-20\right)^{2}}=\sqrt{0}$$

Simplify.

$$x-20=0 x-20=0$$

Add 20 to both sides of the equation.

$$x=20 x=20$$

The equation is now solved. Solutions are the same.

$$x=20$$