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Exercise 11.13 · Q7

Q.∫sin⁡3x dx\displaystyle\int \sin^3 x\,dx is

(1) −34cos⁡x−cos⁡3x12+c\dfrac{-3}4\cos x-\dfrac{\cos 3x}{12}+c
(2) 34cos⁡x+cos⁡3x12+c\dfrac34\cos x+\dfrac{\cos 3x}{12}+c
(3) −34cos⁡x+cos⁡3x12+c\dfrac{-3}4\cos x+\dfrac{\cos 3x}{12}+c
(4) −34sin⁡x−sin⁡3x12+c\dfrac{-3}4\sin x-\dfrac{\sin 3x}{12}+c
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Reduce the power using the triple-angle identity, then integrate term by term.

Step 1. Use sin⁡3x=3sin⁡x−4sin⁡3x ⇒ sin⁡3x=3sin⁡x−sin⁡3x4\sin 3x=3\sin x-4\sin^3x\ \Rightarrow\ \sin^3x=\dfrac{3\sin x-\sin 3x}4. …

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