Solve 2x^2+84x=0

$$2 x ^ { 2 } + 84 x = 0$$

Solve for x

$$x=-42$$

$$x=0$$

Steps Using Factoring

Factor out x.

$$x\left(2x+84\right)=0$$

To find equation solutions, solve x=0 and 2x+84=0.

$$x=0 x=-42$$

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.

$$2x^{2}+84x=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 2 for a, 84 for b, and 0 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-84±\sqrt{84^{2}}}{2\times 2}$$

Take the square root of 84^{2}.

$$x=\frac{-84±84}{2\times 2}$$

Multiply 2 times 2.

$$x=\frac{-84±84}{4}$$

Now solve the equation x=\frac{-84±84}{4} when ± is plus. Add -84 to 84.

$$x=\frac{0}{4}$$

Divide 0 by 4.

$$x=0$$

Now solve the equation x=\frac{-84±84}{4} when ± is minus. Subtract 84 from -84.

$$x=-\frac{168}{4}$$

Divide -168 by 4.

$$x=-42$$

The equation is now solved.

$$x=0 x=-42$$

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.

$$2x^{2}+84x=0$$

Divide both sides by 2.

$$\frac{2x^{2}+84x}{2}=\frac{0}{2}$$

Dividing by 2 undoes the multiplication by 2.

$$x^{2}+\frac{84}{2}x=\frac{0}{2}$$

Divide 84 by 2.

$$x^{2}+42x=\frac{0}{2}$$

Divide 0 by 2.

$$x^{2}+42x=0$$

Divide 42, the coefficient of the x term, by 2 to get 21. Then add the square of 21 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+42x+21^{2}=21^{2}$$

Square 21.

$$x^{2}+42x+441=441$$

Factor x^{2}+42x+441. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+21\right)^{2}=441$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+21\right)^{2}}=\sqrt{441}$$

Simplify.

$$x+21=21 x+21=-21$$

Subtract 21 from both sides of the equation.

$$x=0 x=-42$$