Solve x^2+36x+84=0

$$x ^ { 2 } + 36 x + 84 = 0$$

Solve for x

$$x=4\sqrt{15}-18\approx -2.508066615$$

$$x=-4\sqrt{15}-18\approx -33.491933385$$

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.

$$x^{2}+36x+84=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, 36 for b, and 84 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-36±\sqrt{36^{2}-4\times 84}}{2}$$

Square 36.

$$x=\frac{-36±\sqrt{1296-4\times 84}}{2}$$

Multiply -4 times 84.

$$x=\frac{-36±\sqrt{1296-336}}{2}$$

Add 1296 to -336.

$$x=\frac{-36±\sqrt{960}}{2}$$

Take the square root of 960.

$$x=\frac{-36±8\sqrt{15}}{2}$$

Now solve the equation x=\frac{-36±8\sqrt{15}}{2} when ± is plus. Add -36 to 8\sqrt{15}.

$$x=\frac{8\sqrt{15}-36}{2}$$

Divide -36+8\sqrt{15} by 2.

$$x=4\sqrt{15}-18$$

Now solve the equation x=\frac{-36±8\sqrt{15}}{2} when ± is minus. Subtract 8\sqrt{15} from -36.

$$x=\frac{-8\sqrt{15}-36}{2}$$

Divide -36-8\sqrt{15} by 2.

$$x=-4\sqrt{15}-18$$

The equation is now solved.

$$x=4\sqrt{15}-18 x=-4\sqrt{15}-18$$

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.

$$x^{2}+36x+84=0$$

Subtract 84 from both sides of the equation.

$$x^{2}+36x+84-84=-84$$

Subtracting 84 from itself leaves 0.

$$x^{2}+36x=-84$$

Divide 36, the coefficient of the x term, by 2 to get 18. Then add the square of 18 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+36x+18^{2}=-84+18^{2}$$

Square 18.

$$x^{2}+36x+324=-84+324$$

Add -84 to 324.

$$x^{2}+36x+324=240$$

Factor x^{2}+36x+324. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+18\right)^{2}=240$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+18\right)^{2}}=\sqrt{240}$$

Simplify.

$$x+18=4\sqrt{15} x+18=-4\sqrt{15}$$

Subtract 18 from both sides of the equation.

$$x=4\sqrt{15}-18 x=-4\sqrt{15}-18$$

Steps Using Direct Factoring Method

Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form x^2+Bx+C=0.

$$x ^ 2 +36x +84 = 0$$

Let r and s be the factors for the quadratic equation such that x^2+Bx+C=(x−r)(x−s) where sum of factors (r+s)=−B and the product of factors rs = C

$$r + s = -36 rs = 84$$

Two numbers r and s sum up to -36 exactly when the average of the two numbers is \frac{1}{2}*-36 = -18. You can also see that the midpoint of r and s corresponds to the axis of symmetry of the parabola represented by the quadratic equation y=x^2+Bx+C. The values of r and s are equidistant from the center by an unknown quantity u. Express r and s with respect to variable u. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = -18 - u s = -18 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 84

$$(-18 - u) (-18 + u) = 84$$

Simplify by expanding (a -b) (a + b) = a^2 – b^2

$$324 - u^2 = 84$$

Simplify the expression by subtracting 324 on both sides

$$-u^2 = 84-324 = -240$$

Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u

$$u^2 = 240 u = \pm\sqrt{240} = \pm \sqrt{240}$$

The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.

$$r =-18 - \sqrt{240} = -33.492 s = -18 + \sqrt{240} = -2.508$$