
in the bigger triangle sin @ = R / (L2+ R2)1/2 ....................(A)
cos @ = L / (L2+ R2)1/2 ....................(B)
Now the area of the required triangle (the shaded area) = b X h
in smaller triangle, with angle @ .... h = L cos @ and b = L sin @
therefore, the area = L2 sin @ cos @ substituting values from (A) and (B)
= RL3 / (L2+ R2)