   

^zz dx dx

^yy dy dy

^zz dz dz

+ K  dΨ dΦ + dΨ dΦ + K  dΨ dΦ + dΨ dΦ

yz  dy dz dz dy  zx  dz dx dx dz 

• K  dΨ dΦ + dΨ dΦdxdydz

xy  dx dy dy dx 

= Φ[(K l + K m + k n) dΨ + (K l + K m + K n) dΨ

∫ ∫ xx yx zx dx xy yy zy dy

• (K l + K m + K n) dΨds −

Φ d

 K dΨ + K dΨ

xz yz zz

dz 