We have \sin \, ax + \cos \, bx = \frac{1}{2} (\sin (a+b) x + \sin (a-b) x
Knowing that
\frac{d}{dx} \ cos \, nx = - n \sin \, nx
Hence \int (\sin \, ax \cos \, bx) dx = \int (\frac{1}{2} (\sin (a+b) x + \sin (a-b) x)dx
= \frac{\cos(a+b)x}{a+b} - \frac{\cos(a-b)x}{a-b} + C
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