Consider three unit vectors OA, OB and OC drawn from the origin to the vertices of the triangle (on the unit sphere). The arc BC subtends an angle of magnitude "a" at the centre and therefore OB·OC=cos "a". Introduce a Cartesian basis with OA along the "z"-axis and OB in the "xz"-plane making an angle "c" with the "z"-axis. The vector OC projects to ON in the "xy"-plane and the angle between ON and the "x"-axis is "A". Therefore, the three vectors have components: