Solve 3v^2-36v-39

$$3 v ^ { 2 } - 36 v - 39$$

Factor

$$3\left(v-13\right)\left(v+1\right)$$

Solution Steps

Factor out 3.

$$3\left(v^{2}-12v-13\right)$$

Consider v^{2}-12v-13. Factor the expression by grouping. First, the expression needs to be rewritten as v^{2}+av+bv-13. To find a and b, set up a system to be solved.

$$a+b=-12 ab=1\left(-13\right)=-13$$

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. The only such pair is the system solution.

$$a=-13 b=1$$

Rewrite v^{2}-12v-13 as \left(v^{2}-13v\right)+\left(v-13\right).

$$\left(v^{2}-13v\right)+\left(v-13\right)$$

Factor out v in v^{2}-13v.

$$v\left(v-13\right)+v-13$$

Factor out common term v-13 by using distributive property.

$$\left(v-13\right)\left(v+1\right)$$

Rewrite the complete factored expression.

$$3\left(v-13\right)\left(v+1\right)$$

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.

$$3v^{2}-36v-39=0$$

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.

$$v=\frac{-\left(-36\right)±\sqrt{\left(-36\right)^{2}-4\times 3\left(-39\right)}}{2\times 3}$$

Square -36.

$$v=\frac{-\left(-36\right)±\sqrt{1296-4\times 3\left(-39\right)}}{2\times 3}$$

Multiply -4 times 3.

$$v=\frac{-\left(-36\right)±\sqrt{1296-12\left(-39\right)}}{2\times 3}$$

Multiply -12 times -39.

$$v=\frac{-\left(-36\right)±\sqrt{1296+468}}{2\times 3}$$

Add 1296 to 468.

$$v=\frac{-\left(-36\right)±\sqrt{1764}}{2\times 3}$$

Take the square root of 1764.

$$v=\frac{-\left(-36\right)±42}{2\times 3}$$

The opposite of -36 is 36.

$$v=\frac{36±42}{2\times 3}$$

Multiply 2 times 3.

$$v=\frac{36±42}{6}$$

Now solve the equation v=\frac{36±42}{6} when ± is plus. Add 36 to 42.

$$v=\frac{78}{6}$$

Divide 78 by 6.

$$v=13$$

Now solve the equation v=\frac{36±42}{6} when ± is minus. Subtract 42 from 36.

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

Divide -6 by 6.

$$v=-1$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 13 for x_{1} and -1 for x_{2}.

$$3v^{2}-36v-39=3\left(v-13\right)\left(v-\left(-1\right)\right)$$

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

$$3v^{2}-36v-39=3\left(v-13\right)\left(v+1\right)$$

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 3

$$x ^ 2 -12x -13 = 0$$

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

$$r + s = 12 rs = -13$$

Two numbers r and s sum up to 12 exactly when the average of the two numbers is \frac{1}{2}*12 = 6. 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>

$$r = 6 - u s = 6 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -13

$$(6 - u) (6 + u) = -13$$

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

$$36 - u^2 = -13$$

Simplify the expression by subtracting 36 on both sides

$$-u^2 = -13-36 = -49$$

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

$$u^2 = 49 u = \pm\sqrt{49} = \pm 7$$

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

$$r =6 - 7 = -1 s = 6 + 7 = 13$$

Evaluate

$$3\left(v-13\right)\left(v+1\right)$$