Note that , which follows from continuity. Apply continuity under SS:
Therefore,
Let us make free-body- diagram on CV
(Fig. 14b)
There are pressure forces acting on by the water; reaction forces on CV, Rx and Ry (reaction forces from the support to hold bend in place). Atmospheric pressure acts uniformly over the entire CV, except on where the pressure is (absolute). Therefore, add and substract to make the contribution of atmosphere zero on CV. In other words, .
, which follows from continuity. Apply continuity under SS: 
by the water; reaction forces on CV, Rx and Ry (reaction forces from the support to hold bend in place). Atmospheric pressure acts uniformly over the entire CV, except on 
where the pressure is 
(absolute). Therefore, add and substract 
to make the contribution of atmosphere zero on CV. In other words, 
. 
