Solve x^2-10x-400=0

$${ x }^{ 2 } -10x-400=0$$

Solve for x

$$x=5\sqrt{17}+5\approx 25.615528128$$

$$x=5-5\sqrt{17}\approx -15.615528128$$

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.

$$x^{2}-10x-400=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -10 for b, and -400 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-10\right)±\sqrt{\left(-10\right)^{2}-4\left(-400\right)}}{2}$$

Square -10.

$$x=\frac{-\left(-10\right)±\sqrt{100-4\left(-400\right)}}{2}$$

Multiply -4 times -400.

$$x=\frac{-\left(-10\right)±\sqrt{100+1600}}{2}$$

Add 100 to 1600.

$$x=\frac{-\left(-10\right)±\sqrt{1700}}{2}$$

Take the square root of 1700.

$$x=\frac{-\left(-10\right)±10\sqrt{17}}{2}$$

The opposite of -10 is 10.

$$x=\frac{10±10\sqrt{17}}{2}$$

Now solve the equation x=\frac{10±10\sqrt{17}}{2} when ± is plus. Add 10 to 10\sqrt{17}.

$$x=\frac{10\sqrt{17}+10}{2}$$

Divide 10+10\sqrt{17} by 2.

$$x=5\sqrt{17}+5$$

Now solve the equation x=\frac{10±10\sqrt{17}}{2} when ± is minus. Subtract 10\sqrt{17} from 10.

$$x=\frac{10-10\sqrt{17}}{2}$$

Divide 10-10\sqrt{17} by 2.

$$x=5-5\sqrt{17}$$

The equation is now solved.

$$x=5\sqrt{17}+5 x=5-5\sqrt{17}$$

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.

$$x^{2}-10x-400=0$$

Add 400 to both sides of the equation.

$$x^{2}-10x-400-\left(-400\right)=-\left(-400\right)$$

Subtracting -400 from itself leaves 0.

$$x^{2}-10x=-\left(-400\right)$$

Subtract -400 from 0.

$$x^{2}-10x=400$$

Divide -10, the coefficient of the x term, by 2 to get -5. Then add the square of -5 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-10x+\left(-5\right)^{2}=400+\left(-5\right)^{2}$$

Square -5.

$$x^{2}-10x+25=400+25$$

Add 400 to 25.

$$x^{2}-10x+25=425$$

Factor x^{2}-10x+25. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-5\right)^{2}=425$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-5\right)^{2}}=\sqrt{425}$$

Simplify.

$$x-5=5\sqrt{17} x-5=-5\sqrt{17}$$

Add 5 to both sides of the equation.

$$x=5\sqrt{17}+5 x=5-5\sqrt{17}$$