Solve 3r^2-24r=-45

$$3 r ^ { 2 } - 24 r = - 45$$

Solve for r

$$r=3$$

$$r=5$$

Steps Using Factoring By Grouping

Add 45 to both sides.

$$3r^{2}-24r+45=0$$

Divide both sides by 3.

$$r^{2}-8r+15=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as r^{2}+ar+br+15. To find a and b, set up a system to be solved.

$$a+b=-8 ab=1\times 15=15$$

Since ab is positive, a and b have the same sign. Since a+b is negative, a and b are both negative. List all such integer pairs that give product 15.

$$-1,-15 -3,-5$$

Calculate the sum for each pair.

$$-1-15=-16 -3-5=-8$$

The solution is the pair that gives sum -8.

$$a=-5 b=-3$$

Rewrite r^{2}-8r+15 as \left(r^{2}-5r\right)+\left(-3r+15\right).

$$\left(r^{2}-5r\right)+\left(-3r+15\right)$$

Factor out r in the first and -3 in the second group.

$$r\left(r-5\right)-3\left(r-5\right)$$

Factor out common term r-5 by using distributive property.

$$\left(r-5\right)\left(r-3\right)$$

To find equation solutions, solve r-5=0 and r-3=0.

$$r=5 r=3$$

Steps Using the Quadratic Formula

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.

$$3r^{2}-24r=-45$$

Add 45 to both sides of the equation.

$$3r^{2}-24r-\left(-45\right)=-45-\left(-45\right)$$

Subtracting -45 from itself leaves 0.

$$3r^{2}-24r-\left(-45\right)=0$$

Subtract -45 from 0.

$$3r^{2}-24r+45=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 3 for a, -24 for b, and 45 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$r=\frac{-\left(-24\right)±\sqrt{\left(-24\right)^{2}-4\times 3\times 45}}{2\times 3}$$

Square -24.

$$r=\frac{-\left(-24\right)±\sqrt{576-4\times 3\times 45}}{2\times 3}$$

Multiply -4 times 3.

$$r=\frac{-\left(-24\right)±\sqrt{576-12\times 45}}{2\times 3}$$

Multiply -12 times 45.

$$r=\frac{-\left(-24\right)±\sqrt{576-540}}{2\times 3}$$

Add 576 to -540.

$$r=\frac{-\left(-24\right)±\sqrt{36}}{2\times 3}$$

Take the square root of 36.

$$r=\frac{-\left(-24\right)±6}{2\times 3}$$

The opposite of -24 is 24.

$$r=\frac{24±6}{2\times 3}$$

Multiply 2 times 3.

$$r=\frac{24±6}{6}$$

Now solve the equation r=\frac{24±6}{6} when ± is plus. Add 24 to 6.

$$r=\frac{30}{6}$$

Divide 30 by 6.

$$r=5$$

Now solve the equation r=\frac{24±6}{6} when ± is minus. Subtract 6 from 24.

$$r=\frac{18}{6}$$

Divide 18 by 6.

$$r=3$$

The equation is now solved.

$$r=5 r=3$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$3r^{2}-24r=-45$$

Divide both sides by 3.

$$\frac{3r^{2}-24r}{3}=-\frac{45}{3}$$

Dividing by 3 undoes the multiplication by 3.

$$r^{2}+\left(-\frac{24}{3}\right)r=-\frac{45}{3}$$

Divide -24 by 3.

$$r^{2}-8r=-\frac{45}{3}$$

Divide -45 by 3.

$$r^{2}-8r=-15$$

Divide -8, the coefficient of the x term, by 2 to get -4. Then add the square of -4 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$r^{2}-8r+\left(-4\right)^{2}=-15+\left(-4\right)^{2}$$

Square -4.

$$r^{2}-8r+16=-15+16$$

Add -15 to 16.

$$r^{2}-8r+16=1$$

Factor r^{2}-8r+16. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(r-4\right)^{2}=1$$

Take the square root of both sides of the equation.

$$\sqrt{\left(r-4\right)^{2}}=\sqrt{1}$$

Simplify.

$$r-4=1 r-4=-1$$

Add 4 to both sides of the equation.

$$r=5 r=3$$