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Math Help - Determine x-int of func

  1. #1
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    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
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  2. #2
    Junior Member mathfun's Avatar
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    the discriminant of the equation is $$D = {\left( { - 8} \right)^2} - 4\left( { - 18} \right) = 64 + 72 = 136 > 0$$

    Roots

    $${x_1} = \tfrac{{8 + \sqrt {136} }}{2},{x_2} = \tfrac{{8 - \sqrt {136} }}{2}$$
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  3. #3
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    e^(i*pi)'s Avatar
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    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
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