4 sin S. sin S-a. sin S-b. sin S-c sin' C . (4.) All these forms for the determination of an angle when the three sides are given, are convenient for computation by logarithms. When only one angle is required, the equation (2.) will generally be most appropriate; and when two or all the angles are to be found, the equation (3.) will be the most expeditious, since the same numbers are used in the three calculations (c). (90.) Suppose two sides and the contained angle are given, to find the other angles and the third side. gives the value of A, and their difference that of B. The third side c, is determined from (85). (91.) The third side, however, may be found without knowing the two angles; that is directly from the data, and this may be done in the following manner. By (88.), we have, cos c=cos a. cos b+sin a. sin b. cos C; cos c=cos a. cos b+2 sin a. sin b. |