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Thread: calc quadratic?!?!?

  1. #1
    Aug 2008

    calc quadratic?!?!?

    heyya, got an exam in a few days... just going over old exam papers and found.

    x = 1-e^-t (sin3t + 3cos3t)
    dx/dt = 10e^-t sin(3t)
    d^x/dt^2 = e^-t (3cos3t - sin3t)

    show that: d2x/dt2 + 2dx/dt + 10x = 10
    got a Latex image of it because i know formulea are hard to read like that^^

    anyway... never seen a calculus question like it (also i did the working for dx/dt and d2x/dt2 so maybe errors) and i was hoping someone could point me in the right direction.

    > maybe a name used, if it were in a textbook what would the title be
    > any good resources from mathsworld or wikibooks math etc... on this type of question
    > or if you have the time specific help with this question =]

    thanks for any help

    p.s. first post... im new here XD
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  2. #2
    Super Member
    Sep 2007
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  3. #3
    MHF Contributor

    Aug 2008
    Paris, France
    Actually, you don't need to know anything about differential equations.

    You computed $\displaystyle \frac{dx}{dt}$ and $\displaystyle \frac{d^2x}{dt^2}$ (warning: you forgot a factor 10 in the latter). The whole you are asked to do now is to compute the sum $\displaystyle \frac{d^2x}{dt^2}+2\frac{dx}{dt}+10x$ and to check that you get 10. Which is the case indeed.
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