When to use the law of cosines
Case 1
~ Given: Two sides and the angle between them - SAS
~ Find: The third side.
Case 2
~ Given: All three sides - SSS
~ Find: The three angles
Special steps for Case 2:
~ After you find one angle (using the law of cosines), use the law of sines.
~ Don't use the law of cosines again because it is standard practice to use the law of sines.
Remember, you must first multiply 2 × a × b × cos(angle) before subtracting.
Below are the three formulas, but it is worth your time and effort to remember the first.