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Thread: Please help me with a problem on derivatives and differentiability?

  1. #1
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    Please help me with a problem on derivatives and differentiability?

    Hello everyone. Attached is a screenshot of one of the questions given to us in our calculus homework and I attempted to solve it but seemed to have failed at it... The question says "perpendicular," so I went through some of my old notes and found that the rule is to get the negative inverse of the slope. The negative inverse of the given equation is supposed to be 1, but when I tried solving, the slope I arrived at is -2. Sorry if I'm not making any sense here...

    Please point me in the right direction. Thank you so much!
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  2. #2
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    Re: Please help me with a problem on derivatives and differentiability?

    Quote Originally Posted by kaonashi View Post
    Hello everyone. Attached is a screenshot of one of the questions given to us in our calculus homework and I attempted to solve it but seemed to have failed at it... The question says "perpendicular," so I went through some of my old notes and found that the rule is to get the negative inverse of the slope. The negative inverse of the given equation is supposed to be 1, but when I tried solving, the slope I arrived at is -2. Sorry if I'm not making any sense here...

    Please point me in the right direction. Thank you so much!
    The slope of the line y = -x +1 is -1. Therefore the slope of a line perp to that is 1 (neg recip as you said).

    You now have to find the point on the given parabola where the slope of the tangent is 1, and hence find the equation of that tangent.

    (If you show how you got -2, then I could help you more. I am wondering if you differentiated and subbed in x=1 ??)
    Thanks from kaonashi
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  3. #3
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    Re: Please help me with a problem on derivatives and differentiability?

    The derivative is the slope of the tangent line and, if y= f(x), so that y'= f'(x), it is y' that is the derivative, not x. Once you have that y'= -6x+ 4, set y', not x, equal to 1 and solve for x.
    Last edited by HallsofIvy; Feb 9th 2019 at 11:39 AM.
    Thanks from topsquark and kaonashi
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