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calculus 1 hw problem!?

i have noooo idea how to do this, there must be some trick to do it, i appreciate any help

thanks in advance!

Find the 85th derivative of y = cos(3x).

Update:

the first derivative of y = cos(3x)

y' = -sin(3x) * 3

= -3 sin(3x)

i'm absolutely sure of that one...

3 Answers

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  • 1 decade ago
    Favorite Answer

    y´=-3sin3x

    y´´=-3^2 cos3x

    y´´´= 3^3 sinx

    y´´´´= 3^4 cos3x

    so after four steps you return to cos 3x multiplied by 3^4

    so as84 is a multiple of 4

    y(84) = 3^84 cos 3x and

    y(85) = -3^85sin3x

  • 1 decade ago

    I'm going to assume that you can derive y = cos(3x) regularly.

    y' = -9^1 cos(3x)

    y'' = 9^2 cos(3x)

    y'''= -9^3 cos(3x)

    y''''= 9^4 cos(3x)

    etc.

    From that pattern, you can determine that

    y to the nth prime = (-9)^n cos(3x)

    so y to the 85th prime = (-9)^85 cos(3x)

  • 5 years ago

    ln(2^n) = n * ln(2), so that you've this sequence: ? a million / ln(2^n) = ? a million / (n * ln(2)) = (a million / ln(2)) * ? a million/n. because the quotient (a million / ln(2)) is only a authentic volume, your sequence converges if (and on condition that) ? a million/n does. besides the undeniable fact that it would not converge, and so neither does your sequence.

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