**Corollary 5.1b**: The area of the isosceles triangle in the
figure

is

or

** **

**proof 1**: Half of the base is csin(a) or ccos(b). The height
is ccos(a) or csin(b). In either event the area is either
c^{2}sin(a)cos(a) or c^{2}sin(b)cos(b).

**proof 2**: By Theorem 5.1a, the area is
c^{2}sin(2a)/2. But sin(2a) = 2sin(a)cos(a). The other result
follows from the fact that sin(b) = cos(a) and sin(a) = cos(b).