Solve 8x^2+2x-15

$$8 x ^ { 2 } + 2 x - 15$$

Factor

$$\left(4x-5\right)\left(2x+3\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 8x^{2}+ax+bx-15. To find a and b, set up a system to be solved.

$$a+b=2 ab=8\left(-15\right)=-120$$

Since ab is negative, a and b have the opposite signs. Since a+b is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product -120.

$$-1,120 -2,60 -3,40 -4,30 -5,24 -6,20 -8,15 -10,12$$

Calculate the sum for each pair.

$$-1+120=119 -2+60=58 -3+40=37 -4+30=26 -5+24=19 -6+20=14 -8+15=7 -10+12=2$$

The solution is the pair that gives sum 2.

$$a=-10 b=12$$

Rewrite 8x^{2}+2x-15 as \left(8x^{2}-10x\right)+\left(12x-15\right).

$$\left(8x^{2}-10x\right)+\left(12x-15\right)$$

Factor out 2x in the first and 3 in the second group.

$$2x\left(4x-5\right)+3\left(4x-5\right)$$

Factor out common term 4x-5 by using distributive property.

$$\left(4x-5\right)\left(2x+3\right)$$

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.

$$8x^{2}+2x-15=0$$

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.

$$x=\frac{-2±\sqrt{2^{2}-4\times 8\left(-15\right)}}{2\times 8}$$

Square 2.

$$x=\frac{-2±\sqrt{4-4\times 8\left(-15\right)}}{2\times 8}$$

Multiply -4 times 8.

$$x=\frac{-2±\sqrt{4-32\left(-15\right)}}{2\times 8}$$

Multiply -32 times -15.

$$x=\frac{-2±\sqrt{4+480}}{2\times 8}$$

Add 4 to 480.

$$x=\frac{-2±\sqrt{484}}{2\times 8}$$

Take the square root of 484.

$$x=\frac{-2±22}{2\times 8}$$

Multiply 2 times 8.

$$x=\frac{-2±22}{16}$$

Now solve the equation x=\frac{-2±22}{16} when ± is plus. Add -2 to 22.

$$x=\frac{20}{16}$$

Reduce the fraction \frac{20}{16} to lowest terms by extracting and canceling out 4.

$$x=\frac{5}{4}$$

Now solve the equation x=\frac{-2±22}{16} when ± is minus. Subtract 22 from -2.

$$x=-\frac{24}{16}$$

Reduce the fraction \frac{-24}{16} to lowest terms by extracting and canceling out 8.

$$x=-\frac{3}{2}$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute \frac{5}{4} for x_{1} and -\frac{3}{2} for x_{2}.

$$8x^{2}+2x-15=8\left(x-\frac{5}{4}\right)\left(x-\left(-\frac{3}{2}\right)\right)$$

Simplify all the expressions of the form p-\left(-q\right) to p+q.

$$8x^{2}+2x-15=8\left(x-\frac{5}{4}\right)\left(x+\frac{3}{2}\right)$$

Subtract \frac{5}{4} from x by finding a common denominator and subtracting the numerators. Then reduce the fraction to lowest terms if possible.

$$8x^{2}+2x-15=8\times \frac{4x-5}{4}\left(x+\frac{3}{2}\right)$$

Add \frac{3}{2} to x by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$8x^{2}+2x-15=8\times \frac{4x-5}{4}\times \frac{2x+3}{2}$$

Multiply \frac{4x-5}{4} times \frac{2x+3}{2} by multiplying numerator times numerator and denominator times denominator. Then reduce the fraction to lowest terms if possible.

$$8x^{2}+2x-15=8\times \frac{\left(4x-5\right)\left(2x+3\right)}{4\times 2}$$

Multiply 4 times 2.

$$8x^{2}+2x-15=8\times \frac{\left(4x-5\right)\left(2x+3\right)}{8}$$

Cancel out 8, the greatest common factor in 8 and 8.

$$8x^{2}+2x-15=\left(4x-5\right)\left(2x+3\right)$$

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

$$x ^ 2 +\frac{1}{4}x -\frac{15}{8} = 0$$

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

$$r + s = -\frac{1}{4} rs = -\frac{15}{8}$$

Two numbers r and s sum up to -\frac{1}{4} exactly when the average of the two numbers is \frac{1}{2}*-\frac{1}{4} = -\frac{1}{8}. 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>

$$r = -\frac{1}{8} - u s = -\frac{1}{8} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{15}{8}

$$(-\frac{1}{8} - u) (-\frac{1}{8} + u) = -\frac{15}{8}$$

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

$$\frac{1}{64} - u^2 = -\frac{15}{8}$$

Simplify the expression by subtracting \frac{1}{64} on both sides

$$-u^2 = -\frac{15}{8}-\frac{1}{64} = -\frac{121}{64}$$

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

$$u^2 = \frac{121}{64} u = \pm\sqrt{\frac{121}{64}} = \pm \frac{11}{8}$$

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

$$r =-\frac{1}{8} - \frac{11}{8} = -1.500 s = -\frac{1}{8} + \frac{11}{8} = 1.250$$

Evaluate

$$\left(4x-5\right)\left(2x+3\right)$$