Solve 8x^2-7x-20

$$8 x ^ { 2 } - 7 x - 20$$

Factor

$$8\left(x-\frac{7-\sqrt{689}}{16}\right)\left(x-\frac{\sqrt{689}+7}{16}\right)$$

Steps Using the Quadratic Formula

Quadratic polynomial can be factored using the transformation ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right), where x_{1} and x_{2} are the solutions of the quadratic equation ax^{2}+bx+c=0.

$$8x^{2}-7x-20=0$$

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.

$$x=\frac{-\left(-7\right)±\sqrt{\left(-7\right)^{2}-4\times 8\left(-20\right)}}{2\times 8}$$

Square -7.

$$x=\frac{-\left(-7\right)±\sqrt{49-4\times 8\left(-20\right)}}{2\times 8}$$

Multiply -4 times 8.

$$x=\frac{-\left(-7\right)±\sqrt{49-32\left(-20\right)}}{2\times 8}$$

Multiply -32 times -20.

$$x=\frac{-\left(-7\right)±\sqrt{49+640}}{2\times 8}$$

Add 49 to 640.

$$x=\frac{-\left(-7\right)±\sqrt{689}}{2\times 8}$$

The opposite of -7 is 7.

$$x=\frac{7±\sqrt{689}}{2\times 8}$$

Multiply 2 times 8.

$$x=\frac{7±\sqrt{689}}{16}$$

Now solve the equation x=\frac{7±\sqrt{689}}{16} when ± is plus. Add 7 to \sqrt{689}.

$$x=\frac{\sqrt{689}+7}{16}$$

Now solve the equation x=\frac{7±\sqrt{689}}{16} when ± is minus. Subtract \sqrt{689} from 7.

$$x=\frac{7-\sqrt{689}}{16}$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute \frac{7+\sqrt{689}}{16} for x_{1} and \frac{7-\sqrt{689}}{16} for x_{2}.

$$8x^{2}-7x-20=8\left(x-\frac{\sqrt{689}+7}{16}\right)\left(x-\frac{7-\sqrt{689}}{16}\right)$$

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 8

$$x ^ 2 -\frac{7}{8}x -\frac{5}{2} = 0$$

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

$$r + s = \frac{7}{8} rs = -\frac{5}{2}$$

Two numbers r and s sum up to \frac{7}{8} exactly when the average of the two numbers is \frac{1}{2}*\frac{7}{8} = \frac{7}{16}. 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>

$$r = \frac{7}{16} - u s = \frac{7}{16} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{5}{2}

$$(\frac{7}{16} - u) (\frac{7}{16} + u) = -\frac{5}{2}$$

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

$$\frac{49}{256} - u^2 = -\frac{5}{2}$$

Simplify the expression by subtracting \frac{49}{256} on both sides

$$-u^2 = -\frac{5}{2}-\frac{49}{256} = -\frac{689}{256}$$

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

$$u^2 = \frac{689}{256} u = \pm\sqrt{\frac{689}{256}} = \pm \frac{\sqrt{689}}{16}$$

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

$$r =\frac{7}{16} - \frac{\sqrt{689}}{16} = -1.203 s = \frac{7}{16} + \frac{\sqrt{689}}{16} = 2.078$$

Evaluate

$$8x^{2}-7x-20$$