用分部积分法求∫[(secx)^3]dx

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用分部积分法求∫[(secx)^3]dx

用分部积分法求∫[(secx)^3]dx
用分部积分法求∫[(secx)^3]dx

用分部积分法求∫[(secx)^3]dx
这是一道用分部积分法做的非常著名的题目.
∫[(secx)^3]dx
=∫secx d(tanx)
=secxtanx-∫secxtan²xdx
=secxtanx-∫secx(sec²x-1)dx
=secxtanx-∫sec³xdx+∫secxdx
=secxtanx+ln|secx+tanx|-∫sec³xdx
∫sec³xdx=(1/2)[secxtanx+ln|secx+tanx|]+c

设I = ∫ sec³x dx
= ∫ secx * (sec²x dx) = ∫ secx d(tanx)
= secxtanx - ∫ tanx d(secx)
= secxtanx - ∫ tanx * (secxtanx dx)
= secxtanx - ∫ secxtan²x dx
= secxtanx - ∫ sec...

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设I = ∫ sec³x dx
= ∫ secx * (sec²x dx) = ∫ secx d(tanx)
= secxtanx - ∫ tanx d(secx)
= secxtanx - ∫ tanx * (secxtanx dx)
= secxtanx - ∫ secxtan²x dx
= secxtanx - ∫ secx(sec²x - 1) dx
= secxtanx - I + ∫ secx dx
2I = secxtanx + ∫ secx dx
I = (1/2)secxtanx + (1/2)∫ secx dx
= (1/2)secxtanx + (1/2)ln|secx + tanx| + C

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