Solve x^2+x-20==

$${ x }^{ 2 } +x-20==$$

Factor

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

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx-20. To find a and b, set up a system to be solved.

$$a+b=1 ab=1\left(-20\right)=-20$$

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 -20.

$$-1,20 -2,10 -4,5$$

Calculate the sum for each pair.

$$-1+20=19 -2+10=8 -4+5=1$$

The solution is the pair that gives sum 1.

$$a=-4 b=5$$

Rewrite x^{2}+x-20 as \left(x^{2}-4x\right)+\left(5x-20\right).

$$\left(x^{2}-4x\right)+\left(5x-20\right)$$

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

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

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

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

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.

$$x^{2}+x-20=0$$

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.

$$x=\frac{-1±\sqrt{1^{2}-4\left(-20\right)}}{2}$$

Square 1.

$$x=\frac{-1±\sqrt{1-4\left(-20\right)}}{2}$$

Multiply -4 times -20.

$$x=\frac{-1±\sqrt{1+80}}{2}$$

Add 1 to 80.

$$x=\frac{-1±\sqrt{81}}{2}$$

Take the square root of 81.

$$x=\frac{-1±9}{2}$$

Now solve the equation x=\frac{-1±9}{2} when ± is plus. Add -1 to 9.

$$x=\frac{8}{2}$$

Divide 8 by 2.

$$x=4$$

Now solve the equation x=\frac{-1±9}{2} when ± is minus. Subtract 9 from -1.

$$x=-\frac{10}{2}$$

Divide -10 by 2.

$$x=-5$$

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

$$x^{2}+x-20=\left(x-4\right)\left(x-\left(-5\right)\right)$$

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

$$x^{2}+x-20=\left(x-4\right)\left(x+5\right)$$

Evaluate

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