Solve 20y-y^2=72

$$20 y - y ^ { 2 } = 72$$

Solve for y

$$y=2\sqrt{7}+10\approx 15.291502622$$

$$y=10-2\sqrt{7}\approx 4.708497378$$

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.

$$-y^{2}+20y=72$$

Subtract 72 from both sides of the equation.

$$-y^{2}+20y-72=72-72$$

Subtracting 72 from itself leaves 0.

$$-y^{2}+20y-72=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 20 for b, and -72 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$y=\frac{-20±\sqrt{20^{2}-4\left(-1\right)\left(-72\right)}}{2\left(-1\right)}$$

Square 20.

$$y=\frac{-20±\sqrt{400-4\left(-1\right)\left(-72\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$y=\frac{-20±\sqrt{400+4\left(-72\right)}}{2\left(-1\right)}$$

Multiply 4 times -72.

$$y=\frac{-20±\sqrt{400-288}}{2\left(-1\right)}$$

Add 400 to -288.

$$y=\frac{-20±\sqrt{112}}{2\left(-1\right)}$$

Take the square root of 112.

$$y=\frac{-20±4\sqrt{7}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$y=\frac{-20±4\sqrt{7}}{-2}$$

Now solve the equation y=\frac{-20±4\sqrt{7}}{-2} when ± is plus. Add -20 to 4\sqrt{7}.

$$y=\frac{4\sqrt{7}-20}{-2}$$

Divide -20+4\sqrt{7} by -2.

$$y=10-2\sqrt{7}$$

Now solve the equation y=\frac{-20±4\sqrt{7}}{-2} when ± is minus. Subtract 4\sqrt{7} from -20.

$$y=\frac{-4\sqrt{7}-20}{-2}$$

Divide -20-4\sqrt{7} by -2.

$$y=2\sqrt{7}+10$$

The equation is now solved.

$$y=10-2\sqrt{7} y=2\sqrt{7}+10$$

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.

$$-y^{2}+20y=72$$

Divide both sides by -1.

$$\frac{-y^{2}+20y}{-1}=\frac{72}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$y^{2}+\frac{20}{-1}y=\frac{72}{-1}$$

Divide 20 by -1.

$$y^{2}-20y=\frac{72}{-1}$$

Divide 72 by -1.

$$y^{2}-20y=-72$$

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.

$$y^{2}-20y+\left(-10\right)^{2}=-72+\left(-10\right)^{2}$$

Square -10.

$$y^{2}-20y+100=-72+100$$

Add -72 to 100.

$$y^{2}-20y+100=28$$

Factor y^{2}-20y+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}.

$$\left(y-10\right)^{2}=28$$

Take the square root of both sides of the equation.

$$\sqrt{\left(y-10\right)^{2}}=\sqrt{28}$$

Simplify.

$$y-10=2\sqrt{7} y-10=-2\sqrt{7}$$

Add 10 to both sides of the equation.

$$y=2\sqrt{7}+10 y=10-2\sqrt{7}$$