 The cosine law is used in order to solve non-90° triangles where the sine law won't work. These are recognized by the missing side being across from the given angle, or when solving for the angle, all three sides are known. 
Finding a Missing Side Example 1: Find side X 
x2 = a2 + b2 - 2·a·b·cosθ x2 = 162 + 202 - 2·16·20·cos30 x2 = 101.74 x = 10.1 rounded to 1 d.p. The other sides can now be found using the sine law. Example 2: Find side X 
x2 = a2 + b2 - 2·a·b·cosθ x2 = 202 + 302 - 2·20·30·cos150 x2 = 1819.62 x = 42.7 rounded to 1 d.p. The other sides can now be found using the sine law. Let's use the second formula to find a missing angle >>> |