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    why can't you make sinx=u and use this substitution to do it?
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    (Original post by runny4)
    why can't you make sinx=u and use this substitution to do it?
    Well, you'd get

    du = \cos x \ dx

    There's the problem.
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    Hint: You may wish to consider the following trigonometric identity.

    \displaystyle \cos (2x)=1-2\sin ^2(x).
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    (Original post by Thediplomat56)
    Attachment 473295

    Sorry it's meant to be -1/4sin2theta
    Don't post full solutions.
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    (Original post by Thediplomat56)
    Attachment 473295

    Sorry it's meant to be -1/4sin2theta
    Edit: WHoops thought you were the OP. Stupid 16Characters.
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    (Original post by runny4)
    why can't you make sinx=u and use this substitution to do it?
    Well, what do you end up with when you try this?
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    (Original post by Indeterminate)
    Well, you'd get

    du = \cos x \ dx

    There's the problem.
    why is that a problem?- you end up with something in terms of sin and cos
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    (Original post by davros)
    Well, what do you end up with when you try this?
    you end up with sin cubed x over 3cosx
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    (Original post by runny4)
    you end up with sin cubed x over 3cosx
    Presumably you had an integral of

     \displaystyle \int \frac{u^2}{cos x} du

    The issue is that  cos x = cos(arcsin (u)) so you cannot treat it as a constant when integrating w.r.t u.
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    (Original post by 16Characters....)
    Presumably you had an integral of

     \displaystyle \int \frac{u^2}{cos x} du

    The issue is that  cos x = cos(arcsin (u)) so you cannot treat it as a constant when integrating w.r.t u.
    ok thank you i thought that you could treat cosx as constant with respect to u and bring it to the front of the integral but obviously u can't.
 
 
 
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