第1个回答 2019-12-07
O(0,0) A(1,1) B(1,0)
∮L xy ds= ∮OA xyds +∮AB xyds +∮BO xyds
∮AB xyds =0 ∮BO xyds=0
∮L xy ds= ∮OA xyds =∫(0,1) x^3[1+(x^2)'^2]^(1/2)dx
=∫(0,1) x^3(1+4x^2)^(1/2)dx
令 x=1/2tant
∫ x^3(1+4x^2)^(1/2)dx
=1/16 ∫ tan^3tsec^3tdt
=1/16 ∫ tan^2tsect^2t dsect
=1/16 ∫ (sec^2t-1)sec^2t dsect
=1/16 ∫ (sec^4t-sec^2t ) dsect
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