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Math Help - x coordinates of horizontal tangents lines ?

  1. #1
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    Question x coordinates of horizontal tangents lines ?

    I'm not sure if I posed the question correctly but the problem is: find the x coordinates of the points onthe graph of y=x^3-6x^2-15X+1 where the tangent lines are horizontal ?

    First step:
    Take the derivative which is 3x^2-12x-15

    Second step:
    Set the equation to zero (because that is the slope of the horizontal line)

    Third Step:
    What am I solving for ?

    LOST !!!!

    I tried to graph it in my calculator and draw the tangent lines to get a clue but no such luck
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  2. #2
    Member Henderson's Avatar
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    You've got this figured out yourself:

    Quote Originally Posted by kbgemini16 View Post
    I'm not sure if I posed the question correctly but the problem is: find the x coordinates of the points onthe graph of y=x^3-6x^2-15X+1 where the tangent lines are horizontal ?

    First step:
    Take the derivative which is 3x^2-12x-15

    Second step:
    Set the equation to zero (because that is the slope of the horizontal line)

    Third Step:
    What am I solving for ?

    LOST !!!!

    I tried to graph it in my calculator and draw the tangent lines to get a clue but no such luck


    3x^2-12x-15=0

    Now solve for x.
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