z = xye^z， 两边对 x 求偏导，得
z'<x> = ye^z + xyz'<x>e^z (1)
则 z'<x> = ye^z/(1-xye^z)
式（1） 再对 x 求偏导，得
z''<xx> = yz'<x>e^z + yz'<x>e^z+xyz''<x>e^z+xy(z'<x>)^2e^z
则 z'<xx> = yz'<x>e^z (2+xz'<x>)/(1-xye^z)
= y^2e^(2z) [2(1-xye^z)+xye^z]/(1-xye^z)^3
= y^2e^(2z) (2-xye^z)/(1-xye^z)^3
z'<x> = ye^z + xyz'<x>e^z (1)
则 z'<x> = ye^z/(1-xye^z)
式（1） 再对 x 求偏导，得
z''<xx> = yz'<x>e^z + yz'<x>e^z+xyz''<x>e^z+xy(z'<x>)^2e^z
则 z'<xx> = yz'<x>e^z (2+xz'<x>)/(1-xye^z)
= y^2e^(2z) [2(1-xye^z)+xye^z]/(1-xye^z)^3
= y^2e^(2z) (2-xye^z)/(1-xye^z)^3
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