du/dx, du/dy, du/dz, (d^2)u/dxdz
[m]\frac{ ∂ u}{ ∂ x}=u`_{x}=(x^{3}+2y^2-4z^5+x\cdot cos2z)`_{x}=3x^2+cos2z[/m]
[m]\frac{ ∂ u}{ ∂ x}=u`_{y}=(x^{3}+2y^2-4z^5+x\cdot cos2z)`_{y}= 4y [/m]
[m]\frac{ ∂ u}{ ∂z}=u`_{z}=(x^{3}+2y^2-4z^5+x\cdot cos2z)`_{z}=-20x^4-2xsin2z [/m]
[m]\frac{ ∂^2 u}{ ∂ x ∂z }=(u`_{x})`_{z}=(3x^2+cos2z)`_{z}=(-sin2z)\cdot (2z)`=-2sin2z[/m]