# Thread: Determine x-int of func

1. ## Determine x-int of func

Determine x-intercept of function. $f(x) = x^2 -8x -18$
I know I need to set to zero. $x^2 -8x -18 =0$
Isolate for x. $x(x-8) -18 =0$

2. the discriminant of the equation is $D = {\left( { - 8} \right)^2} - 4\left( { - 18} \right) = 64 + 72 = 136 > 0$

Roots

${x_1} = \tfrac{{8 + \sqrt {136} }}{2},{x_2} = \tfrac{{8 - \sqrt {136} }}{2}$

3. Indeed you do set it to 0 to get it in the form $ax^2+bx+c=0$.

Since $a$ and $c$ have different sign then there will be two real roots (ie: $b^2-4ac>0$ which is what mathfun showed). This won't factor (136 is not square) and so use the quadratic formula.

For the purposes of simplifying the square root: $136 = 2^3 \cdot 17$