_Let_ S _and_ s _be the sines of two sides of a spherical triangle_, d _the sine of half the difference of the same sides_, a _the sine of half the included angle_, b _the sine of half the base; and writing unity for the radius, we have_ Ssa² + d² - b² = 0; _in which_ a _or_ b _may be made the unknown quantity, as the case requires_.