My Math Forum Integral of sin(x)*cos(x)

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 September 9th, 2017, 04:55 AM #1 Member   Joined: Sep 2013 Posts: 84 Thanks: 2 Integral of sin(x)*cos(x) Hello I don't understand why I get different results when I try to find that integral. If I choose f' as cos(x) and g as sin(x) then using the partial integration formula the result is (1/2)*sin^2(x). If I choose f' as sin(x) and g as cos(x), then the result is -(1/2)*cos^2(x). The two results are not equal, so something is wrong (the 2nd result is correct). Last edited by skipjack; September 9th, 2017 at 12:22 PM.
 September 9th, 2017, 06:06 AM #2 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,331 Thanks: 2457 Math Focus: Mainly analysis and algebra Recall that $\sin^2 x + \cos^2 x = 1$ so the two results differ only by an additive constant. Thus they have the same derivative and are therefore both primitives of the same function (your integrand). Note that an easier way to get a (third different) result is to use $\sin 2x = 2\sin x \cos x$ to simplify the integrand before you integrate. Thanks from ricsi046
 September 9th, 2017, 06:45 AM #3 Member   Joined: Sep 2013 Posts: 84 Thanks: 2 oh,I see,thanks Thanks from v8archie

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