Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find parametric equations for the tangent line to the curve at t=1

  1. Oct 20, 2004 #1
    Original question:

    a) Say r'(t) = 3t^2 i - cost j + 2t k, and r(0) = i + k. Find r(t).
    b) Find T(t).
    c) Find parametric equations for the tangent line to the curve at t=1.

    I have done parts a and b and got the following results:

    a) r(t) = t^3 + 1 i - sint j + t^2 + 1 k
    b)T(t) = r'(t)/|r'(t)| = (3t^2 i - cost j + 2t k)/(sqrt(9t^4 + cos^2(t) + 4t^2))

    Are these answers correct so far? I'm unsure about my answer for T(t) because the denominator seems so messy.

    Also, and most importantly, how would I do part (c)?

  2. jcsd
  3. Oct 21, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    The component of the k-vector should be a primitive function of 2t. I`m sure this was a silly mistake, since you got it right for the i-vector.

    T(t) is correct. It's just a matter of plugging in your equations.

    For c), use T(t) and plug in t=1. That gives you the unit tangent, so you know the direction of the line. Then given that the line passes through the point r(1) you have enough info to determine the line.
  4. Oct 21, 2004 #3


    User Avatar
    Science Advisor

    It would be a good idea to put in parentheses:

    r(t)= (t3+ 1)i- (sin t)j+ (t2+ 1)k

    Assuming that by T(t) you mean the unit tangent vector, then your answer is correct- divide r' by its own length.

    (c) is easy: you don't need the unit tangent vector, just "a" tangent vector and r' works nicely:

    x= 3t2+ 1
    y= -cos t
    z= 2t+ 1

    The "+1" terms are, of course, from r(0)= i+ k.
  5. Nov 7, 2004 #4

    I don't know how to make new threads! Well anyway, could someone help me with my parametric equation?
  6. Nov 8, 2004 #5


    User Avatar
    Science Advisor

    Go back to the list of threads. At the bottom is a "new thread" button. As far as your "parametric equation" is concerned, we can't help you if you don't post it!
  7. Nov 8, 2004 #6


    User Avatar
    Science Advisor

    Of course, those should be

    x= 3t+ 1
    y= -cos(1)t
    z= 2t+ 1
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook