Evaluate: `int1/(sinx+sin2x)\ dx`

Explanation

`int frac{1}{sinx-sin2x}dx`

`=int frac{dx}{sinx-2sinxcosx}`

`=int frac{sinx}{(1-cos^2x)(1-2cosx)}`

`cos x = t` or `-sin x dx = dt` or `sin x dx = -dt`, we get

`=int frac{-dt}{(1-t^2)(1-2t)}`

`=int frac{-1}{(1+t)(1-t)(1-2t)}dt`

`=frac{A}{1+t}+frac{B}{1-t}+frac{C}{1-2t}`

`A=1/6`, `B=-1/2` and `C=4/3`
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