第2个回答 2011-04-28
∫e^x*cosxdx
=∫e^xdsinx
=e^x*sinx-∫sinxd(e^x)
=e^x*sinx-∫e^x*sinxdx
=e^x*sinx+∫e^x*(-sinx)dx
=e^x*sinx+∫e^x*dcosx
=e^x*sinx+e^x*cosx-∫cosxd(e^x)
=e^x*sinx+e^x*cosx-∫e^x*cosxdx,
=e^x(sinx+cosx)-∫e^x*cosxdx,
2∫e^x*cosxdx=e^x(sinx+cosx)+2C(C为任意常数),
∫e^x*cosxdx=[e^x(sinx+cosx)]/2+C.
e^x>0,
当x=π/2或x=3π/2时,cosx=0,e^x*cosx=0.
当0<=x=<π/2或3π/2<x<=2π时,cosx>0,e^x*cosx>0.
当π/2<x<3π/2时,cosx<0,e^x*cosx<0
曲线y=e^x * cosx, 0 <=x<=2π,与x轴围成的图形的面积
=∫<0,π/2>e^x*cosxdx+∫<π/2,3π/2>[-e^x*cosx]dx+∫<3π/2,2π>e^x*cosxdx
=∫<0,π/2>e^x*cosxdx+∫<3π/2,π/2>e^x*cosxdx+∫<3π/2,2π>e^x*cosxdx
=∫<0,π/2>e^x*cosxdx+∫<3π/2,π/2>e^x*cosxdx+∫<3π/2,2π>e^x*cosxdx
=[e^x(sinx+cosx)]/2|<0,π/2>+[e^x(sinx+cosx)]/2|<3π/2,π/2>+[e^x(sinx+cosx)]/2|<3π/2,2π>
=e^(π/2)-1/2+e^(π/2)+e^(3π/2)+e^(2π)+e^(3π/2)
=2e^(π/2)+2e^(3π/2)+e^(2π)-1/2本回答被提问者采纳