The two shaded regions above the Axis of Symmetry (Red Line) will always be treated as a single triangle.  The shaded region below this axis will be treated as a separate triangle. Both of these triangles share a common dimension, namely, the Tail's portion of the Axis of Symmetry, labled "a".
The angle "C" at the apex of the Light Cone is split into two equall angles each of which is labled "c". Being in different triangles, each angle "c" will have a different Side Opposite. Although those are labled c and c' their positions indicate their status as sides rather than angles, and the shading in the following images make it clear which part of which triangle is appropriate.  To avoid rounding errors, all calculations are performed to 10 place accuracy and displayed rounded off to four decimal places.