设Ω:a≤x≤b,c≤y≤d,ι≤z≤m,证明:
∫∫∫Ωf1(x)•f2(y)•f3(z)dxdydz=
(∫abf1(x)dx)•(∫cdf2(y)dy)•
(∫ιmf3(z)dz).
证明:由题意知: ∫∫∫Ωf1(x)f2(y)f3(z)dxdydz= ∫abdx∫cddy∫ιm f1(x)f2(y)f3(z)dz =∫abf1(x)dx∫cdf2(y)dy∫em f3(z)dz ∴f1(x)f2(y)f3(z)dxdydz=(∫abf1(x)dx) (∫cdf2(y)dy)(∫ιmf3(z)dz)