Solve 3x^2-27x-30

$$3 { x }^{ 2 } -27x-30$$

Factor

$$3\left(x-10\right)\left(x+1\right)$$

Solution Steps

Factor out 3.

$$3\left(x^{2}-9x-10\right)$$

Consider x^{2}-9x-10. Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx-10. To find a and b, set up a system to be solved.

$$a+b=-9 ab=1\left(-10\right)=-10$$

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 -10.

$$1,-10 2,-5$$

Calculate the sum for each pair.

$$1-10=-9 2-5=-3$$

The solution is the pair that gives sum -9.

$$a=-10 b=1$$

Rewrite x^{2}-9x-10 as \left(x^{2}-10x\right)+\left(x-10\right).

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

Factor out x in x^{2}-10x.

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

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

$$\left(x-10\right)\left(x+1\right)$$

Rewrite the complete factored expression.

$$3\left(x-10\right)\left(x+1\right)$$

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.

$$3x^{2}-27x-30=0$$

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.

$$x=\frac{-\left(-27\right)±\sqrt{\left(-27\right)^{2}-4\times 3\left(-30\right)}}{2\times 3}$$

Square -27.

$$x=\frac{-\left(-27\right)±\sqrt{729-4\times 3\left(-30\right)}}{2\times 3}$$

Multiply -4 times 3.

$$x=\frac{-\left(-27\right)±\sqrt{729-12\left(-30\right)}}{2\times 3}$$

Multiply -12 times -30.

$$x=\frac{-\left(-27\right)±\sqrt{729+360}}{2\times 3}$$

Add 729 to 360.

$$x=\frac{-\left(-27\right)±\sqrt{1089}}{2\times 3}$$

Take the square root of 1089.

$$x=\frac{-\left(-27\right)±33}{2\times 3}$$

The opposite of -27 is 27.

$$x=\frac{27±33}{2\times 3}$$

Multiply 2 times 3.

$$x=\frac{27±33}{6}$$

Now solve the equation x=\frac{27±33}{6} when ± is plus. Add 27 to 33.

$$x=\frac{60}{6}$$

Divide 60 by 6.

$$x=10$$

Now solve the equation x=\frac{27±33}{6} when ± is minus. Subtract 33 from 27.

$$x=-\frac{6}{6}$$

Divide -6 by 6.

$$x=-1$$

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 -1 for x_{2}.

$$3x^{2}-27x-30=3\left(x-10\right)\left(x-\left(-1\right)\right)$$

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

$$3x^{2}-27x-30=3\left(x-10\right)\left(x+1\right)$$

Evaluate

$$3\left(x-10\right)\left(x+1\right)$$