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Math Help - Integeral: Trig Substitution help

  1. #1
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    Integeral: Trig Substitution help



    I have this equation

    So I refer to a triangle and use these values for substitution.

    = 5 cos(x)

    x = 5 sin(x)

    dx = 5 sin(x) = 5 cos(x)

    I ended up having something like this.



    I canceled the 5 sin(x)s leaving me with

    integral(5 cos x) = - 5 sin x + c

    I looked at the triangles and got back.

    x/5 + c

    Then I looked back at the book for answers and I was way way way off.

    What did I do that was totally wrong?
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  2. #2
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    hey man i dont think u set up your triangle properly.. it should be (25-x^2)^1/2 divided by 5
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  3. #3
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    a = sqrt(25-x^2)
    b = x
    c = 5

    ^ I thought that is how you normally set it up for a sqrt(c^2 - x^2) situation where c is a constant.
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  4. #4
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    No that's wrong. You solved for it. Cos=x/5 so adjacent is x and 5 is the hypotnuse using pythagorems theorm you can solve for the other side
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