Solve x^2-3x-70

$${ x }^{ 2 } -3x-70$$

Factor

$$\left(x-10\right)\left(x+7\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx-70. To find a and b, set up a system to be solved.

$$a+b=-3 ab=1\left(-70\right)=-70$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -70.

$$1,-70 2,-35 5,-14 7,-10$$

Calculate the sum for each pair.

$$1-70=-69 2-35=-33 5-14=-9 7-10=-3$$

The solution is the pair that gives sum -3.

$$a=-10 b=7$$

Rewrite x^{2}-3x-70 as \left(x^{2}-10x\right)+\left(7x-70\right).

$$\left(x^{2}-10x\right)+\left(7x-70\right)$$

Factor out x in the first and 7 in the second group.

$$x\left(x-10\right)+7\left(x-10\right)$$

Factor out common term x-10 by using distributive property.

$$\left(x-10\right)\left(x+7\right)$$

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.

$$x^{2}-3x-70=0$$

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.

$$x=\frac{-\left(-3\right)±\sqrt{\left(-3\right)^{2}-4\left(-70\right)}}{2}$$

Square -3.

$$x=\frac{-\left(-3\right)±\sqrt{9-4\left(-70\right)}}{2}$$

Multiply -4 times -70.

$$x=\frac{-\left(-3\right)±\sqrt{9+280}}{2}$$

Add 9 to 280.

$$x=\frac{-\left(-3\right)±\sqrt{289}}{2}$$

Take the square root of 289.

$$x=\frac{-\left(-3\right)±17}{2}$$

The opposite of -3 is 3.

$$x=\frac{3±17}{2}$$

Now solve the equation x=\frac{3±17}{2} when ± is plus. Add 3 to 17.

$$x=\frac{20}{2}$$

Divide 20 by 2.

$$x=10$$

Now solve the equation x=\frac{3±17}{2} when ± is minus. Subtract 17 from 3.

$$x=-\frac{14}{2}$$

Divide -14 by 2.

$$x=-7$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 10 for x_{1} and -7 for x_{2}.

$$x^{2}-3x-70=\left(x-10\right)\left(x-\left(-7\right)\right)$$

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

$$x^{2}-3x-70=\left(x-10\right)\left(x+7\right)$$

Evaluate

$$\left(x-10\right)\left(x+7\right)$$