I really don't get the substitution rule. This is supposed to be the easiest problem in the homework set: u=3x(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\int {\cos \,3x\,\,dx\,\, = \,\,\int {\cos \,u\,\, = \,\,\sin \,u + C\,\, = \,\,\sin 3x + C} } [/tex]

But the right answer is 1/3 sin(3x). Where did the 1/3 come from?

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# I really don't get the substitution rule

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