Solve 7x^2+10x-8

$$7 x ^ { 2 } + 10 x - 8$$

Factor

$$\left(7x-4\right)\left(x+2\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 7x^{2}+ax+bx-8. To find a and b, set up a system to be solved.

$$a+b=10 ab=7\left(-8\right)=-56$$

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 -56.

$$-1,56 -2,28 -4,14 -7,8$$

Calculate the sum for each pair.

$$-1+56=55 -2+28=26 -4+14=10 -7+8=1$$

The solution is the pair that gives sum 10.

$$a=-4 b=14$$

Rewrite 7x^{2}+10x-8 as \left(7x^{2}-4x\right)+\left(14x-8\right).

$$\left(7x^{2}-4x\right)+\left(14x-8\right)$$

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

$$x\left(7x-4\right)+2\left(7x-4\right)$$

Factor out common term 7x-4 by using distributive property.

$$\left(7x-4\right)\left(x+2\right)$$

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.

$$7x^{2}+10x-8=0$$

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.

$$x=\frac{-10±\sqrt{10^{2}-4\times 7\left(-8\right)}}{2\times 7}$$

Square 10.

$$x=\frac{-10±\sqrt{100-4\times 7\left(-8\right)}}{2\times 7}$$

Multiply -4 times 7.

$$x=\frac{-10±\sqrt{100-28\left(-8\right)}}{2\times 7}$$

Multiply -28 times -8.

$$x=\frac{-10±\sqrt{100+224}}{2\times 7}$$

Add 100 to 224.

$$x=\frac{-10±\sqrt{324}}{2\times 7}$$

Take the square root of 324.

$$x=\frac{-10±18}{2\times 7}$$

Multiply 2 times 7.

$$x=\frac{-10±18}{14}$$

Now solve the equation x=\frac{-10±18}{14} when ± is plus. Add -10 to 18.

$$x=\frac{8}{14}$$

Reduce the fraction \frac{8}{14} to lowest terms by extracting and canceling out 2.

$$x=\frac{4}{7}$$

Now solve the equation x=\frac{-10±18}{14} when ± is minus. Subtract 18 from -10.

$$x=-\frac{28}{14}$$

Divide -28 by 14.

$$x=-2$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute \frac{4}{7} for x_{1} and -2 for x_{2}.

$$7x^{2}+10x-8=7\left(x-\frac{4}{7}\right)\left(x-\left(-2\right)\right)$$

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

$$7x^{2}+10x-8=7\left(x-\frac{4}{7}\right)\left(x+2\right)$$

Subtract \frac{4}{7} from x by finding a common denominator and subtracting the numerators. Then reduce the fraction to lowest terms if possible.

$$7x^{2}+10x-8=7\times \frac{7x-4}{7}\left(x+2\right)$$

Cancel out 7, the greatest common factor in 7 and 7.

$$7x^{2}+10x-8=\left(7x-4\right)\left(x+2\right)$$

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 7

$$x ^ 2 +\frac{10}{7}x -\frac{8}{7} = 0$$

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

$$r + s = -\frac{10}{7} rs = -\frac{8}{7}$$

Two numbers r and s sum up to -\frac{10}{7} exactly when the average of the two numbers is \frac{1}{2}*-\frac{10}{7} = -\frac{5}{7}. 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>

$$r = -\frac{5}{7} - u s = -\frac{5}{7} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{8}{7}

$$(-\frac{5}{7} - u) (-\frac{5}{7} + u) = -\frac{8}{7}$$

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

$$\frac{25}{49} - u^2 = -\frac{8}{7}$$

Simplify the expression by subtracting \frac{25}{49} on both sides

$$-u^2 = -\frac{8}{7}-\frac{25}{49} = -\frac{81}{49}$$

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

$$u^2 = \frac{81}{49} u = \pm\sqrt{\frac{81}{49}} = \pm \frac{9}{7}$$

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

$$r =-\frac{5}{7} - \frac{9}{7} = -2 s = -\frac{5}{7} + \frac{9}{7} = 0.571$$

Evaluate

$$\left(7x-4\right)\left(x+2\right)$$