u = x^(y^z)
全微分:
du = d(x^(y^z))
= y^z * x^(y^z - 1) dx + x^(y^z) * Lnx * z *y^(z -1) dy + x^(y^z) * Lnx *y^z * Lny dz
偏微分:
u'x = y^z * x^(y^z - 1) = 或者 = y^z * x^(y^z) / x
u'y = x^(y^z) * Lnx * z *y^(z -1) = 或者 = x^(y^z) * Lnx * y^z * z / y
u'z = x^(y^z) * Lnx *y^z * Lny = 或者 = x^(y^z) * y^z * * Lnx * Lny