高数。。第二累曲线积分，第五题求解救。。
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x=cost y=1 z=sint (1,1,0)对应t=0, (0,1,1)对应t=π/2
F=(-kx/(x^2+y^2),-ky/(x^2+y^2),0)
W=-∫[L]kx/(x^2+y^2)dx+ky/(x^2+y^2)dy
=∫[0,π/2]kcostsintdt/(1+cos^2t)
=k/2∫[0,π/2]sin2tdt/[1+1/2(1+cos2t)]
=k∫[0,π/2]sin2tdt/(3+cos2t)
=-k/2ln(3+cos2t)|[0,π/2]
=2k-k
=k
F=(-kx/(x^2+y^2),-ky/(x^2+y^2),0)
W=-∫[L]kx/(x^2+y^2)dx+ky/(x^2+y^2)dy
=∫[0,π/2]kcostsintdt/(1+cos^2t)
=k/2∫[0,π/2]sin2tdt/[1+1/2(1+cos2t)]
=k∫[0,π/2]sin2tdt/(3+cos2t)
=-k/2ln(3+cos2t)|[0,π/2]
=2k-k
=k
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