Indefinite Integration

Find:

    \[\int \left(\cos^{3}{x}\cdot\cos{3x}+\sin^{3}{x}\cdot\sin{3x}\right)\,\textrm{d}x\]

Source: Vietnam I (Question 7, p. 9)

Author: ascklee

Dr. Lee teaches at the Wee Kim Wee School of Communication and Information at the Nanyang Technological University in Singapore. He founded The Mathematics Digital Library in 2013.

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