The reciprocity relation,

The reciprocity relation, the summation rule, and Equations 13.5 to 13.7 can be used to develop view factor relations that allow for applications of Figure 13.4 and/or 13.6 to more complex configurations. Consider the view factor F14 for surfaces 1 and 4 of the following geometry. These surfaces are perpendicular but do not share a common edge. 
(a) Obtain the following expression for the view factor F14: 
(b) If L1 = L2 = L4 = (W/2) and L3 = W, what is the value of F14?