Solve 3x^2-17x-6

$$3 x ^ { 2 } - 17 x - 6$$

Factor

$$\left(x-6\right)\left(3x+1\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 3x^{2}+ax+bx-6. To find a and b, set up a system to be solved.

$$a+b=-17 ab=3\left(-6\right)=-18$$

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 -18.

$$1,-18 2,-9 3,-6$$

Calculate the sum for each pair.

$$1-18=-17 2-9=-7 3-6=-3$$

The solution is the pair that gives sum -17.

$$a=-18 b=1$$

Rewrite 3x^{2}-17x-6 as \left(3x^{2}-18x\right)+\left(x-6\right).

$$\left(3x^{2}-18x\right)+\left(x-6\right)$$

Factor out 3x in 3x^{2}-18x.

$$3x\left(x-6\right)+x-6$$

Factor out common term x-6 by using distributive property.

$$\left(x-6\right)\left(3x+1\right)$$

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.

$$3x^{2}-17x-6=0$$

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.

$$x=\frac{-\left(-17\right)±\sqrt{\left(-17\right)^{2}-4\times 3\left(-6\right)}}{2\times 3}$$

Square -17.

$$x=\frac{-\left(-17\right)±\sqrt{289-4\times 3\left(-6\right)}}{2\times 3}$$

Multiply -4 times 3.

$$x=\frac{-\left(-17\right)±\sqrt{289-12\left(-6\right)}}{2\times 3}$$

Multiply -12 times -6.

$$x=\frac{-\left(-17\right)±\sqrt{289+72}}{2\times 3}$$

Add 289 to 72.

$$x=\frac{-\left(-17\right)±\sqrt{361}}{2\times 3}$$

Take the square root of 361.

$$x=\frac{-\left(-17\right)±19}{2\times 3}$$

The opposite of -17 is 17.

$$x=\frac{17±19}{2\times 3}$$

Multiply 2 times 3.

$$x=\frac{17±19}{6}$$

Now solve the equation x=\frac{17±19}{6} when ± is plus. Add 17 to 19.

$$x=\frac{36}{6}$$

Divide 36 by 6.

$$x=6$$

Now solve the equation x=\frac{17±19}{6} when ± is minus. Subtract 19 from 17.

$$x=-\frac{2}{6}$$

Reduce the fraction \frac{-2}{6} to lowest terms by extracting and canceling out 2.

$$x=-\frac{1}{3}$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 6 for x_{1} and -\frac{1}{3} for x_{2}.

$$3x^{2}-17x-6=3\left(x-6\right)\left(x-\left(-\frac{1}{3}\right)\right)$$

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

$$3x^{2}-17x-6=3\left(x-6\right)\left(x+\frac{1}{3}\right)$$

Add \frac{1}{3} to x by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$3x^{2}-17x-6=3\left(x-6\right)\times \frac{3x+1}{3}$$

Cancel out 3, the greatest common factor in 3 and 3.

$$3x^{2}-17x-6=\left(x-6\right)\left(3x+1\right)$$

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

$$x ^ 2 -\frac{17}{3}x -2 = 0$$

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

$$r + s = \frac{17}{3} rs = -2$$

Two numbers r and s sum up to \frac{17}{3} exactly when the average of the two numbers is \frac{1}{2}*\frac{17}{3} = \frac{17}{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>

$$r = \frac{17}{6} - u s = \frac{17}{6} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -2

$$(\frac{17}{6} - u) (\frac{17}{6} + u) = -2$$

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

$$\frac{289}{36} - u^2 = -2$$

Simplify the expression by subtracting \frac{289}{36} on both sides

$$-u^2 = -2-\frac{289}{36} = -\frac{361}{36}$$

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

$$u^2 = \frac{361}{36} u = \pm\sqrt{\frac{361}{36}} = \pm \frac{19}{6}$$

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

$$r =\frac{17}{6} - \frac{19}{6} = -0.333 s = \frac{17}{6} + \frac{19}{6} = 6$$

Evaluate

$$\left(x-6\right)\left(3x+1\right)$$