Solve a triangle using the cosine rule: c² = a² + b² − 2ab·cos C.
The Law of Cosines generalises the Pythagorean theorem to any triangle: c² = a² + b² − 2ab·cos C. Use it for SAS (two sides + included angle) or SSS (three sides) configurations.
Law of Cosines
c² = a² + b² − 2ab·cos C
When C = 90°, cos(90°) = 0, so it reduces to c² = a² + b², which is exactly the Pythagorean theorem.
If you know two angles, you know the third (they sum to 180°). The Law of Sines is simpler in that case. Law of Cosines is best for SSS or SAS.