d(X^Y)/dy=X^Y*lnX
U=X^(Y^Z)
U对Z求偏导=[X^(Y^Z)对Y^Z求偏导]*(Y^Z对Z求偏导)
=[X^(Y^Z)*lnX]*(Y^Z*lnY)
=lnX*lnY*Y^Z*X^(Y^Z)