(tanx)^-1/2的原函数

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(tanx)^-1/2的原函数

令t=tanx,则dx=dt/(1+t²) ∫dx/√tanx=∫dt/(√t(1+t²)),再令t=u²,则dt=2udu,∫dx/√tanx=∫dt/(√t(1+t²))=2∫du/(1+u^4)= ∫[(u^2+1)-(u^2-1)]/(1+u^4)dx
= {∫(u^2+1)/(1+u^4) dx - ∫(u^2-1)/(1+u^4)dx }
= {∫(1+1/u^2)dx /(u^2+1/u^2) - ∫(1-1/u^2)dx/(u^2+1/u^2)}
= {∫d(u-u/x) /[(u-1/u)^2+2] - ∫d(u+1/u) /[(u+1/u)^2 -2] }
= { 1/√2 ∫d[(u-1/u) /√2] /{[(u-1/u)/√2]^2+1} - ∫d(u+1/u) /[(u+1/u)^2 -2] }
= { 1/√2 ∫d[(u-1/u) /√2] /{[(u-1/u)/√2]^2+1}
- 1/2√2 ∫d[(u+1/u) /√2] [ 1/{[(u+1/u)/√2] -1} - 1/{[(u+1/u)/√2] +1 }]
= √2/2*arctan[(u-1/u)/√2] - √2/4*ln|(u^2-u√2+1)/(u^2+u√2 +1)| + C
=√2/2*arctan[(√tanx-1/√tanx)/√2] - √2/4*ln|(tanx-√(2tanx)+1)/(tanx+√(2tanx) +1)| + C