Solve 5x^2+17x-40

$$5 x ^ { 2 } + 17 x - 40$$

Factor

$$\left(5x-8\right)\left(x+5\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 5x^{2}+ax+bx-40. To find a and b, set up a system to be solved.

$$a+b=17 ab=5\left(-40\right)=-200$$

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 -200.

$$-1,200 -2,100 -4,50 -5,40 -8,25 -10,20$$

Calculate the sum for each pair.

$$-1+200=199 -2+100=98 -4+50=46 -5+40=35 -8+25=17 -10+20=10$$

The solution is the pair that gives sum 17.

$$a=-8 b=25$$

Rewrite 5x^{2}+17x-40 as \left(5x^{2}-8x\right)+\left(25x-40\right).

$$\left(5x^{2}-8x\right)+\left(25x-40\right)$$

Factor out x in the first and 5 in the second group.

$$x\left(5x-8\right)+5\left(5x-8\right)$$

Factor out common term 5x-8 by using distributive property.

$$\left(5x-8\right)\left(x+5\right)$$

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.

$$5x^{2}+17x-40=0$$

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.

$$x=\frac{-17±\sqrt{17^{2}-4\times 5\left(-40\right)}}{2\times 5}$$

Square 17.

$$x=\frac{-17±\sqrt{289-4\times 5\left(-40\right)}}{2\times 5}$$

Multiply -4 times 5.

$$x=\frac{-17±\sqrt{289-20\left(-40\right)}}{2\times 5}$$

Multiply -20 times -40.

$$x=\frac{-17±\sqrt{289+800}}{2\times 5}$$

Add 289 to 800.

$$x=\frac{-17±\sqrt{1089}}{2\times 5}$$

Take the square root of 1089.

$$x=\frac{-17±33}{2\times 5}$$

Multiply 2 times 5.

$$x=\frac{-17±33}{10}$$

Now solve the equation x=\frac{-17±33}{10} when ± is plus. Add -17 to 33.

$$x=\frac{16}{10}$$

Reduce the fraction \frac{16}{10} to lowest terms by extracting and canceling out 2.

$$x=\frac{8}{5}$$

Now solve the equation x=\frac{-17±33}{10} when ± is minus. Subtract 33 from -17.

$$x=-\frac{50}{10}$$

Divide -50 by 10.

$$x=-5$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute \frac{8}{5} for x_{1} and -5 for x_{2}.

$$5x^{2}+17x-40=5\left(x-\frac{8}{5}\right)\left(x-\left(-5\right)\right)$$

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

$$5x^{2}+17x-40=5\left(x-\frac{8}{5}\right)\left(x+5\right)$$

Subtract \frac{8}{5} from x by finding a common denominator and subtracting the numerators. Then reduce the fraction to lowest terms if possible.

$$5x^{2}+17x-40=5\times \frac{5x-8}{5}\left(x+5\right)$$

Cancel out 5, the greatest common factor in 5 and 5.

$$5x^{2}+17x-40=\left(5x-8\right)\left(x+5\right)$$

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 5

$$x ^ 2 +\frac{17}{5}x -8 = 0$$

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

$$r + s = -\frac{17}{5} rs = -8$$

Two numbers r and s sum up to -\frac{17}{5} exactly when the average of the two numbers is \frac{1}{2}*-\frac{17}{5} = -\frac{17}{10}. 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>

$$r = -\frac{17}{10} - u s = -\frac{17}{10} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -8

$$(-\frac{17}{10} - u) (-\frac{17}{10} + u) = -8$$

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

$$\frac{289}{100} - u^2 = -8$$

Simplify the expression by subtracting \frac{289}{100} on both sides

$$-u^2 = -8-\frac{289}{100} = -\frac{1089}{100}$$

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

$$u^2 = \frac{1089}{100} u = \pm\sqrt{\frac{1089}{100}} = \pm \frac{33}{10}$$

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

$$r =-\frac{17}{10} - \frac{33}{10} = -5 s = -\frac{17}{10} + \frac{33}{10} = 1.600$$

Evaluate

$$\left(5x-8\right)\left(x+5\right)$$