\w xpwx (.i) - wxpwx (x + dx)]dydz =
. , , 5wxPwx j s j j SwxPwx
= wxpwx dy dz — ywxpwx -t-----dx) dy dz =--:——dv
8x
dx
[<7= (x) - u „ (x + dx)]dydz =
= arxx dy dz-(<7xx + ^—SLdx)dydz = - dV 8x ' 8x
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