Solve -x^2-4x=-16

$$- { x }^{ 2 } -4x=-16$$

Solve for x

$$x=2\sqrt{5}-2\approx 2.472135955$$

$$x=-2\sqrt{5}-2\approx -6.472135955$$

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.

$$-x^{2}-4x=-16$$

Add 16 to both sides of the equation.

$$-x^{2}-4x-\left(-16\right)=-16-\left(-16\right)$$

Subtracting -16 from itself leaves 0.

$$-x^{2}-4x-\left(-16\right)=0$$

Subtract -16 from 0.

$$-x^{2}-4x+16=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, -4 for b, and 16 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-4\right)±\sqrt{\left(-4\right)^{2}-4\left(-1\right)\times 16}}{2\left(-1\right)}$$

Square -4.

$$x=\frac{-\left(-4\right)±\sqrt{16-4\left(-1\right)\times 16}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-\left(-4\right)±\sqrt{16+4\times 16}}{2\left(-1\right)}$$

Multiply 4 times 16.

$$x=\frac{-\left(-4\right)±\sqrt{16+64}}{2\left(-1\right)}$$

Add 16 to 64.

$$x=\frac{-\left(-4\right)±\sqrt{80}}{2\left(-1\right)}$$

Take the square root of 80.

$$x=\frac{-\left(-4\right)±4\sqrt{5}}{2\left(-1\right)}$$

The opposite of -4 is 4.

$$x=\frac{4±4\sqrt{5}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{4±4\sqrt{5}}{-2}$$

Now solve the equation x=\frac{4±4\sqrt{5}}{-2} when ± is plus. Add 4 to 4\sqrt{5}.

$$x=\frac{4\sqrt{5}+4}{-2}$$

Divide 4+4\sqrt{5} by -2.

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

Now solve the equation x=\frac{4±4\sqrt{5}}{-2} when ± is minus. Subtract 4\sqrt{5} from 4.

$$x=\frac{4-4\sqrt{5}}{-2}$$

Divide 4-4\sqrt{5} by -2.

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

The equation is now solved.

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

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.

$$-x^{2}-4x=-16$$

Divide both sides by -1.

$$\frac{-x^{2}-4x}{-1}=-\frac{16}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\left(-\frac{4}{-1}\right)x=-\frac{16}{-1}$$

Divide -4 by -1.

$$x^{2}+4x=-\frac{16}{-1}$$

Divide -16 by -1.

$$x^{2}+4x=16$$

Divide 4, the coefficient of the x term, by 2 to get 2. Then add the square of 2 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+4x+2^{2}=16+2^{2}$$

Square 2.

$$x^{2}+4x+4=16+4$$

Add 16 to 4.

$$x^{2}+4x+4=20$$

Factor x^{2}+4x+4. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

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

Take the square root of both sides of the equation.

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

Simplify.

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

Subtract 2 from both sides of the equation.

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