- Functions like ln(x), tan−1(x),
11+x2, sin−1(x),
1√1−x2 and 1x almost
never get grouped into dv. Either we do not
have an (easy) antiderivative for them (like the first
few), so they must be u, or they get a lot simpler
when differentiated or get a lot more complicated when
antidifferentiated.
- Functions like xn (with n>0) get a little
simpler when differentiated and a little more
complicated when antidifferentiated. They
sometimes get grouped into u and sometimes into
dv, depending on what else is in the integrand.
- Functions like sin(x), cos(x) and ex have
the same complexity when differentiated or
antidifferentiated. The other factor in the
integrand will determine whether they should be u or
dv.
- A few functions, like sec2(x), xex2 and
x−n (with n at least 2), actually get simpler
when antidifferentiated. These almost always get
grouped into dv.
- Don't forget that 1 is a function! You can always
apply integration by parts to ∫f(x)dx by
picking u=f(x) and dv=1dx. This can be
used to evaluate ∫ln(x)dx and
∫tan−1(x)dx.
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