Solve 48x^2+720x-2916=0

$$48 x ^ { 2 } + 720 x - 2916 = 0$$

Solve for x

$$x=3\sqrt{13}-\frac{15}{2}\approx 3.316653826$$

$$x=-3\sqrt{13}-\frac{15}{2}\approx -18.316653826$$

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.

$$48x^{2}+720x-2916=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 48 for a, 720 for b, and -2916 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-720±\sqrt{720^{2}-4\times 48\left(-2916\right)}}{2\times 48}$$

Square 720.

$$x=\frac{-720±\sqrt{518400-4\times 48\left(-2916\right)}}{2\times 48}$$

Multiply -4 times 48.

$$x=\frac{-720±\sqrt{518400-192\left(-2916\right)}}{2\times 48}$$

Multiply -192 times -2916.

$$x=\frac{-720±\sqrt{518400+559872}}{2\times 48}$$

Add 518400 to 559872.

$$x=\frac{-720±\sqrt{1078272}}{2\times 48}$$

Take the square root of 1078272.

$$x=\frac{-720±288\sqrt{13}}{2\times 48}$$

Multiply 2 times 48.

$$x=\frac{-720±288\sqrt{13}}{96}$$

Now solve the equation x=\frac{-720±288\sqrt{13}}{96} when ± is plus. Add -720 to 288\sqrt{13}.

$$x=\frac{288\sqrt{13}-720}{96}$$

Divide -720+288\sqrt{13} by 96.

$$x=3\sqrt{13}-\frac{15}{2}$$

Now solve the equation x=\frac{-720±288\sqrt{13}}{96} when ± is minus. Subtract 288\sqrt{13} from -720.

$$x=\frac{-288\sqrt{13}-720}{96}$$

Divide -720-288\sqrt{13} by 96.

$$x=-3\sqrt{13}-\frac{15}{2}$$

The equation is now solved.

$$x=3\sqrt{13}-\frac{15}{2} x=-3\sqrt{13}-\frac{15}{2}$$

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.

$$48x^{2}+720x-2916=0$$

Add 2916 to both sides of the equation.

$$48x^{2}+720x-2916-\left(-2916\right)=-\left(-2916\right)$$

Subtracting -2916 from itself leaves 0.

$$48x^{2}+720x=-\left(-2916\right)$$

Subtract -2916 from 0.

$$48x^{2}+720x=2916$$

Divide both sides by 48.

$$\frac{48x^{2}+720x}{48}=\frac{2916}{48}$$

Dividing by 48 undoes the multiplication by 48.

$$x^{2}+\frac{720}{48}x=\frac{2916}{48}$$

Divide 720 by 48.

$$x^{2}+15x=\frac{2916}{48}$$

Reduce the fraction \frac{2916}{48} to lowest terms by extracting and canceling out 12.

$$x^{2}+15x=\frac{243}{4}$$

Divide 15, the coefficient of the x term, by 2 to get \frac{15}{2}. Then add the square of \frac{15}{2} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+15x+\left(\frac{15}{2}\right)^{2}=\frac{243}{4}+\left(\frac{15}{2}\right)^{2}$$

Square \frac{15}{2} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+15x+\frac{225}{4}=\frac{243+225}{4}$$

Add \frac{243}{4} to \frac{225}{4} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+15x+\frac{225}{4}=117$$

Factor x^{2}+15x+\frac{225}{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}.

$$\left(x+\frac{15}{2}\right)^{2}=117$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{15}{2}\right)^{2}}=\sqrt{117}$$

Simplify.

$$x+\frac{15}{2}=3\sqrt{13} x+\frac{15}{2}=-3\sqrt{13}$$

Subtract \frac{15}{2} from both sides of the equation.

$$x=3\sqrt{13}-\frac{15}{2} x=-3\sqrt{13}-\frac{15}{2}$$

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.This is achieved by dividing both sides of the equation by 48

$$x ^ 2 +15x -\frac{243}{4} = 0$$

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

$$r + s = -15 rs = -\frac{243}{4}$$

Two numbers r and s sum up to -15 exactly when the average of the two numbers is \frac{1}{2}*-15 = -\frac{15}{2}. 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>

$$r = -\frac{15}{2} - u s = -\frac{15}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{243}{4}

$$(-\frac{15}{2} - u) (-\frac{15}{2} + u) = -\frac{243}{4}$$

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

$$\frac{225}{4} - u^2 = -\frac{243}{4}$$

Simplify the expression by subtracting \frac{225}{4} on both sides

$$-u^2 = -\frac{243}{4}-\frac{225}{4} = -117$$

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

$$u^2 = 117 u = \pm\sqrt{117} = \pm \sqrt{117}$$

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

$$r =-\frac{15}{2} - \sqrt{117} = -18.317 s = -\frac{15}{2} + \sqrt{117} = 3.317$$