Shear Stress Formula: Calculating Tau Correctly

[foo9008]

how to get the formula of shear stress (tau) ? i got (-dp/ds -ydz/ds )(r/2) instead of -d(p+yz)/ ds (r/2) as in the book , if it's d(p+yz)/ ds (r/2) m , i would get (-dp/ds -ydz/ds-z(dy/ds) )(r/2) ,, right ?

The Attempt at a Solution

P/s : I'm concerned about the derivation part , so i didnt upload the complete version of the note

 

[foo9008]

Is the book wrong?