Primitive of x by Square of Cosecant of a x

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Theorem

$\ds \int x \csc^2 a x \rd x = \frac {-x \cot a x} a + \frac 1 {a^2} \ln \size {\sin a x} + C$


Proof

With a view to expressing the primitive in the form:

$\ds \int u \frac {\d v} {\d x} \rd x = u v - \int v \frac {\d u} {\d x} \rd x$

let:

\(\ds u\) \(=\) \(\ds x\)
\(\ds \leadsto \ \ \) \(\ds \frac {\d u} {\d x}\) \(=\) \(\ds 1\) Derivative of Identity Function


and let:

\(\ds \frac {\d v} {\d x}\) \(=\) \(\ds \csc^2 a x\)
\(\ds \leadsto \ \ \) \(\ds v\) \(=\) \(\ds \frac {-\cot a x} a\) Primitive of $\csc^2 a x$


Then:

\(\ds \int x \csc^2 a x \rd x\) \(=\) \(\ds \frac {-x \cot a x} a - \int \frac {-\cot a x} a \rd x + C\) Integration by Parts
\(\ds \) \(=\) \(\ds \frac {-x \cot a x} a + \frac 1 a \int \cot a x \rd x + C\) Primitive of Constant Multiple of Function
\(\ds \) \(=\) \(\ds \frac {-x \cot a x} a + \frac 1 a \paren {\frac 1 a \ln \size {\sin a x} } + C\) Primitive of $\cot a x$
\(\ds \) \(=\) \(\ds \frac {-x \cot a x} a + \frac 1 {a^2} \ln \size {\sin a x} + C\) simplifying

$\blacksquare$


Also see


Sources

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