Integration of cos3x

To determine this integral, we will use trigonometric formulas and identity. ∫cos3x dx = ∫cos x . cos2x dx = ∫cos x (1 - sin2x) dx [Why sin2x + cos2x = 1 ⇒ cos2x = 1 - sin2x] = ∫cos x dx - ∫cos x sin2x dx = sin x + C1 - I1, where I1 = dsum = sin2x 3 ∫ For this reason, take 3x = v ⇒ 3dx = ................
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