Solve 9z^2-17z-2

$$9 z ^ { 2 } - 17 z - 2$$

Factor

$$\left(z-2\right)\left(9z+1\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 9z^{2}+az+bz-2. To find a and b, set up a system to be solved.

$$a+b=-17 ab=9\left(-2\right)=-18$$

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 9z^{2}-17z-2 as \left(9z^{2}-18z\right)+\left(z-2\right).

$$\left(9z^{2}-18z\right)+\left(z-2\right)$$

Factor out 9z in 9z^{2}-18z.

$$9z\left(z-2\right)+z-2$$

Factor out common term z-2 by using distributive property.

$$\left(z-2\right)\left(9z+1\right)$$

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.

$$9z^{2}-17z-2=0$$

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.

$$z=\frac{-\left(-17\right)±\sqrt{\left(-17\right)^{2}-4\times 9\left(-2\right)}}{2\times 9}$$

Square -17.

$$z=\frac{-\left(-17\right)±\sqrt{289-4\times 9\left(-2\right)}}{2\times 9}$$

Multiply -4 times 9.

$$z=\frac{-\left(-17\right)±\sqrt{289-36\left(-2\right)}}{2\times 9}$$

Multiply -36 times -2.

$$z=\frac{-\left(-17\right)±\sqrt{289+72}}{2\times 9}$$

Add 289 to 72.

$$z=\frac{-\left(-17\right)±\sqrt{361}}{2\times 9}$$

Take the square root of 361.

$$z=\frac{-\left(-17\right)±19}{2\times 9}$$

The opposite of -17 is 17.

$$z=\frac{17±19}{2\times 9}$$

Multiply 2 times 9.

$$z=\frac{17±19}{18}$$

Now solve the equation z=\frac{17±19}{18} when ± is plus. Add 17 to 19.

$$z=\frac{36}{18}$$

Divide 36 by 18.

$$z=2$$

Now solve the equation z=\frac{17±19}{18} when ± is minus. Subtract 19 from 17.

$$z=-\frac{2}{18}$$

Reduce the fraction \frac{-2}{18} to lowest terms by extracting and canceling out 2.

$$z=-\frac{1}{9}$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 2 for x_{1} and -\frac{1}{9} for x_{2}.

$$9z^{2}-17z-2=9\left(z-2\right)\left(z-\left(-\frac{1}{9}\right)\right)$$

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

$$9z^{2}-17z-2=9\left(z-2\right)\left(z+\frac{1}{9}\right)$$

Add \frac{1}{9} to z by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$9z^{2}-17z-2=9\left(z-2\right)\times \frac{9z+1}{9}$$

Cancel out 9, the greatest common factor in 9 and 9.

$$9z^{2}-17z-2=\left(z-2\right)\left(9z+1\right)$$

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 9

$$x ^ 2 -\frac{17}{9}x -\frac{2}{9} = 0$$

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

$$r + s = \frac{17}{9} rs = -\frac{2}{9}$$

Two numbers r and s sum up to \frac{17}{9} exactly when the average of the two numbers is \frac{1}{2}*\frac{17}{9} = \frac{17}{18}. 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>

$$r = \frac{17}{18} - u s = \frac{17}{18} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{2}{9}

$$(\frac{17}{18} - u) (\frac{17}{18} + u) = -\frac{2}{9}$$

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

$$\frac{289}{324} - u^2 = -\frac{2}{9}$$

Simplify the expression by subtracting \frac{289}{324} on both sides

$$-u^2 = -\frac{2}{9}-\frac{289}{324} = -\frac{361}{324}$$

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

$$u^2 = \frac{361}{324} u = \pm\sqrt{\frac{361}{324}} = \pm \frac{19}{18}$$

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

$$r =\frac{17}{18} - \frac{19}{18} = -0.111 s = \frac{17}{18} + \frac{19}{18} = 2$$

Evaluate

$$\left(z-2\right)\left(9z+1\right)$$