Solve 32x-2x^2-20=0

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

Solve for x

$$x=3\sqrt{6}+8\approx 15.348469228$$

$$x=8-3\sqrt{6}\approx 0.651530772$$

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.

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

This equation is in standard form: ax^{2}+bx+c=0. Substitute -2 for a, 32 for b, and -20 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-32±\sqrt{32^{2}-4\left(-2\right)\left(-20\right)}}{2\left(-2\right)}$$

Square 32.

$$x=\frac{-32±\sqrt{1024-4\left(-2\right)\left(-20\right)}}{2\left(-2\right)}$$

Multiply -4 times -2.

$$x=\frac{-32±\sqrt{1024+8\left(-20\right)}}{2\left(-2\right)}$$

Multiply 8 times -20.

$$x=\frac{-32±\sqrt{1024-160}}{2\left(-2\right)}$$

Add 1024 to -160.

$$x=\frac{-32±\sqrt{864}}{2\left(-2\right)}$$

Take the square root of 864.

$$x=\frac{-32±12\sqrt{6}}{2\left(-2\right)}$$

Multiply 2 times -2.

$$x=\frac{-32±12\sqrt{6}}{-4}$$

Now solve the equation x=\frac{-32±12\sqrt{6}}{-4} when ± is plus. Add -32 to 12\sqrt{6}.

$$x=\frac{12\sqrt{6}-32}{-4}$$

Divide -32+12\sqrt{6} by -4.

$$x=8-3\sqrt{6}$$

Now solve the equation x=\frac{-32±12\sqrt{6}}{-4} when ± is minus. Subtract 12\sqrt{6} from -32.

$$x=\frac{-12\sqrt{6}-32}{-4}$$

Divide -32-12\sqrt{6} by -4.

$$x=3\sqrt{6}+8$$

The equation is now solved.

$$x=8-3\sqrt{6} x=3\sqrt{6}+8$$

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.

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

Add 20 to both sides of the equation.

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

Subtracting -20 from itself leaves 0.

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

Subtract -20 from 0.

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

Divide both sides by -2.

$$\frac{-2x^{2}+32x}{-2}=\frac{20}{-2}$$

Dividing by -2 undoes the multiplication by -2.

$$x^{2}+\frac{32}{-2}x=\frac{20}{-2}$$

Divide 32 by -2.

$$x^{2}-16x=\frac{20}{-2}$$

Divide 20 by -2.

$$x^{2}-16x=-10$$

Divide -16, the coefficient of the x term, by 2 to get -8. Then add the square of -8 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-16x+\left(-8\right)^{2}=-10+\left(-8\right)^{2}$$

Square -8.

$$x^{2}-16x+64=-10+64$$

Add -10 to 64.

$$x^{2}-16x+64=54$$

Factor x^{2}-16x+64. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-8\right)^{2}=54$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-8\right)^{2}}=\sqrt{54}$$

Simplify.

$$x-8=3\sqrt{6} x-8=-3\sqrt{6}$$

Add 8 to both sides of the equation.

$$x=3\sqrt{6}+8 x=8-3\sqrt{6}$$