Primitive of Exponential of a x by Power of Cosine of b x/Lemma 2

Lemma for Primitive of $e^{a x} \cos^n b x$

 * $\dfrac {a^2 + n b^2} a e^{a x} \cos^n b x + \dfrac {n b} a e^{a x} \cos^{n - 1} b x \left({a \sin b x - b \cos b x}\right) = e^{a x} \cos^{n - 1} b x \left({a \cos b x + n b \sin b x}\right)$