Solve -x^2+-10x+15=0

$$- { x }^{ 2 } +-10x+15 = 0$$

Solve for x

$$x=2\sqrt{10}-5\approx 1.32455532$$

$$x=-2\sqrt{10}-5\approx -11.32455532$$

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.

$$-x^{2}-10x+15=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, -10 for b, and 15 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-10\right)±\sqrt{\left(-10\right)^{2}-4\left(-1\right)\times 15}}{2\left(-1\right)}$$

Square -10.

$$x=\frac{-\left(-10\right)±\sqrt{100-4\left(-1\right)\times 15}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-\left(-10\right)±\sqrt{100+4\times 15}}{2\left(-1\right)}$$

Multiply 4 times 15.

$$x=\frac{-\left(-10\right)±\sqrt{100+60}}{2\left(-1\right)}$$

Add 100 to 60.

$$x=\frac{-\left(-10\right)±\sqrt{160}}{2\left(-1\right)}$$

Take the square root of 160.

$$x=\frac{-\left(-10\right)±4\sqrt{10}}{2\left(-1\right)}$$

The opposite of -10 is 10.

$$x=\frac{10±4\sqrt{10}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{10±4\sqrt{10}}{-2}$$

Now solve the equation x=\frac{10±4\sqrt{10}}{-2} when ± is plus. Add 10 to 4\sqrt{10}.

$$x=\frac{4\sqrt{10}+10}{-2}$$

Divide 10+4\sqrt{10} by -2.

$$x=-2\sqrt{10}-5$$

Now solve the equation x=\frac{10±4\sqrt{10}}{-2} when ± is minus. Subtract 4\sqrt{10} from 10.

$$x=\frac{10-4\sqrt{10}}{-2}$$

Divide 10-4\sqrt{10} by -2.

$$x=2\sqrt{10}-5$$

The equation is now solved.

$$x=-2\sqrt{10}-5 x=2\sqrt{10}-5$$

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.

$$-x^{2}-10x+15=0$$

Subtract 15 from both sides of the equation.

$$-x^{2}-10x+15-15=-15$$

Subtracting 15 from itself leaves 0.

$$-x^{2}-10x=-15$$

Divide both sides by -1.

$$\frac{-x^{2}-10x}{-1}=-\frac{15}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\left(-\frac{10}{-1}\right)x=-\frac{15}{-1}$$

Divide -10 by -1.

$$x^{2}+10x=-\frac{15}{-1}$$

Divide -15 by -1.

$$x^{2}+10x=15$$

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.

$$x^{2}+10x+5^{2}=15+5^{2}$$

Square 5.

$$x^{2}+10x+25=15+25$$

Add 15 to 25.

$$x^{2}+10x+25=40$$

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}.

$$\left(x+5\right)^{2}=40$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+5\right)^{2}}=\sqrt{40}$$

Simplify.

$$x+5=2\sqrt{10} x+5=-2\sqrt{10}$$

Subtract 5 from both sides of the equation.

$$x=2\sqrt{10}-5 x=-2\sqrt{10}-5$$