Solve x^2-20x+8000=0

$${ x }^{ 2 } -20x+8000=0$$

Solve for x (complex solution)

$$x=10+10\sqrt{79}i\approx 10+88.881944173i$$

$$x=-10\sqrt{79}i+10\approx 10-88.881944173i$$

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.

$$x^{2}-20x+8000=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -20 for b, and 8000 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-20\right)±\sqrt{\left(-20\right)^{2}-4\times 8000}}{2}$$

Square -20.

$$x=\frac{-\left(-20\right)±\sqrt{400-4\times 8000}}{2}$$

Multiply -4 times 8000.

$$x=\frac{-\left(-20\right)±\sqrt{400-32000}}{2}$$

Add 400 to -32000.

$$x=\frac{-\left(-20\right)±\sqrt{-31600}}{2}$$

Take the square root of -31600.

$$x=\frac{-\left(-20\right)±20\sqrt{79}i}{2}$$

The opposite of -20 is 20.

$$x=\frac{20±20\sqrt{79}i}{2}$$

Now solve the equation x=\frac{20±20\sqrt{79}i}{2} when ± is plus. Add 20 to 20i\sqrt{79}.

$$x=\frac{20+20\sqrt{79}i}{2}$$

Divide 20+20i\sqrt{79} by 2.

$$x=10+10\sqrt{79}i$$

Now solve the equation x=\frac{20±20\sqrt{79}i}{2} when ± is minus. Subtract 20i\sqrt{79} from 20.

$$x=\frac{-20\sqrt{79}i+20}{2}$$

Divide 20-20i\sqrt{79} by 2.

$$x=-10\sqrt{79}i+10$$

The equation is now solved.

$$x=10+10\sqrt{79}i x=-10\sqrt{79}i+10$$

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.

$$x^{2}-20x+8000=0$$

Subtract 8000 from both sides of the equation.

$$x^{2}-20x+8000-8000=-8000$$

Subtracting 8000 from itself leaves 0.

$$x^{2}-20x=-8000$$

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.

$$x^{2}-20x+\left(-10\right)^{2}=-8000+\left(-10\right)^{2}$$

Square -10.

$$x^{2}-20x+100=-8000+100$$

Add -8000 to 100.

$$x^{2}-20x+100=-7900$$

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}.

$$\left(x-10\right)^{2}=-7900$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-10\right)^{2}}=\sqrt{-7900}$$

Simplify.

$$x-10=10\sqrt{79}i x-10=-10\sqrt{79}i$$

Add 10 to both sides of the equation.

$$x=10+10\sqrt{79}i x=-10\sqrt{79}i+10$$