$$ \int \sin \theta\cos \theta~d \theta= \int \frac {1} 2 \sin 2\theta~ d \theta=-\frac {1} 4 \cos 2\theta$$ But, if I let $$ u=\sin \theta , \text{ then }du=\cos \theta~d\theta $$ Then $$ \int \sin \theta\cos \theta~d \theta= \int u ~ du =\frac { u^2 } 2 =\frac {1} 2 \sin^2 \theta $$ Since $$ \sin^2 \theta =\frac {1} 2 - \frac {1} 2 \cos 2\theta$$ The above can be written as $$ \int \sin \theta\cos \theta~d \theta= \frac {1} 2 \sin^2 \theta =\frac {1} 2 \left( \frac {1} 2 - \frac {1} 2 \cos 2\theta \right)=\frac {1} 4-\frac {1} 4 \cos 2\theta $$ Why are the two results differ by the constant $1/4$? Thank you.
Integral of $\sin x \cos x$ using two methods differs by a constant?
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calculus
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0Didn't your calculus instructor drum into your head to always write $+C$ when computing an indefinite integral? I guess your head could use a bit more drumming! – 2012-04-23
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1@Tony : Please notice my edits to your question. If you write 3\sin 5 in $\TeX$, the backslash on \sin not only prevents italicization, but also results in proper spacing before and after $\sin$, so you don't need to insert those spaces yourself. – 2012-04-23
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0@Michael Hardy, thank you very much for the useful info!! – 2012-05-02
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The answer to the indefinite integration is the family of functions, which differ by a constant on every connected area of the domain.
That is, the correct way to write the answer to $\int f(x)dx$ (where $f$ is defined on a continuous area) is $g(x) + C$.
Note that $C-\dfrac{1}{4}\cos{2\theta}$ defines the same family of functions as $C+\dfrac{1}{4}-\dfrac{1}{4}\cos{2\theta}$.
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2A "series" of functions is a potentially confusing word choice, because "series" has a different technical meaning in analysis. Better to speak of a "family" of functions -- or just a "set" of them. – 2012-04-23
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1@HenningMakholm Of course you're right, it is just my knowledge of English which failed me... – 2012-04-23
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0@penartur, Thank you very much for your kind answers and comments!! – 2012-04-23
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0@Henning Makholm, Thank you very much for your comments!! – 2012-04-23
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0@penartur, Again thank you very much for your kind and detailed comments!! – 2012-04-23