Primitive of Square of Hyperbolic Sine of a x over Hyperbolic Cosine of a x

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Theorem

$\ds \int \frac {\sinh^2 a x \rd x} {\cosh a x} = \frac {\sinh a x} a - \frac 1 a \map \arctan {\sinh a x} + C$


Proof

\(\ds \int \frac {\sinh^2 a x \rd x} {\cosh a x}\) \(=\) \(\ds \int \frac {\paren {\cosh^2 a x - 1} \rd x} {\cosh a x}\) Difference of Squares of Hyperbolic Cosine and Sine
\(\ds \) \(=\) \(\ds \int \frac {\paren {\cosh^2 a x} \rd x} {\cosh a x} - \int \frac {\d x} {\cosh a x}\) Linear Combination of Primitives
\(\ds \) \(=\) \(\ds \int \cosh a x \rd x - \int \sech a x \rd x\) Definition 2 of Hyperbolic Secant
\(\ds \) \(=\) \(\ds \frac {\sinh a x} a - \int \sech a x \rd x\) Primitive of $\cosh a x$
\(\ds \) \(=\) \(\ds \frac {\sinh a x} a - \frac 1 a \map \arctan {\sinh a x} + C\) Primitive of $\sech a x$

$\blacksquare$


Sources