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半角代换,令 u = tan(x/2), 则 x = 2arctanu, dx = 2du/(1+u^2),
I = ∫<-tan(π/8), tan(π/8)>[2du/(1+u^2)]/[1+2u/(1+u^2)]
= ∫<-tan(π/8), tan(π/8)>2du/(1+u)^2
= 2∫<-tan(π/8), tan(π/8)>d(1+u)/(1+u)^2
= 2[-1/(1+u)]<-tan(π/8), tan(π/8)>
= 2{1/[1-tan(π/8) - 1/[1+tan(π/8)]}
tan(π/8) = [1-cos(π/4)]/sin(π/4) = (1-1/√2)/(1/√2) = √2-1,
则 I = 2[1/(2-√2) - 1/√2] = 2
I = ∫<-tan(π/8), tan(π/8)>[2du/(1+u^2)]/[1+2u/(1+u^2)]
= ∫<-tan(π/8), tan(π/8)>2du/(1+u)^2
= 2∫<-tan(π/8), tan(π/8)>d(1+u)/(1+u)^2
= 2[-1/(1+u)]<-tan(π/8), tan(π/8)>
= 2{1/[1-tan(π/8) - 1/[1+tan(π/8)]}
tan(π/8) = [1-cos(π/4)]/sin(π/4) = (1-1/√2)/(1/√2) = √2-1,
则 I = 2[1/(2-√2) - 1/√2] = 2
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=∫(1-sinx)/cos²xdx
=∫sec²x-tanxsecxdx
=tanx-secx
=(1-√2)-(-1-√2)
=2
=∫sec²x-tanxsecxdx
=tanx-secx
=(1-√2)-(-1-√2)
=2
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