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Equation of a tangent to a complicated function

  1. Jul 28, 2008 #1
    The problem: The y-intercept of the line tangent to y=(x4-2x2-8)ecosx2 at x=1 is (a) -15.499 (b) -25.999 (c) -41.448 (d) 15.449 (e) 25.99

    To find the y-intercept I have to find the whole equation of the line
    y=f(1)
    m=f'(1)
    x=1
    b=??

    My problem is that I don't think I got the the derivative of f(x) right.
    I know you have to apply the product rule and the chain rule, but I don't know if the chain rule should be applied to ecosx2. I came up with this:
    (4x3-4x)ecosx2 + (x4-2x2-8)ecosx2(-sinx2)(2x)

    then y=-15.4487
    m =15.1465
    and b=-1.020

    the intercept doesn't equal any of the given answers, but y= answer a. However, if I use f'(x)= (4x3-4x)ecosx2 + (x4-2x2-8)ecosx2, I get b=1. What did I do wrong?
     
  2. jcsd
  3. Jul 28, 2008 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Yes, of course, you need to use the chain rule. But I don't see where you got m. When x= 1, the derivative (slope of tangent line) is (4-4)ecos(1)+ (1- 2- 8)ecos(1)(-sin(1))(2)= 18ecos(1)sin(1) and I do NOT get 15.465 for that!
     
  4. Jul 28, 2008 #3
    you're my hero. I made a silly mistake. Thanks so much for catching it!
     
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