EXAMPLE 3 Use the law of cosines in real life SOLUTION b 2 = a 2 + c 2 – 2ac cos B 316 2 = 155 2 + 197 2 – 2(155)(197) cos B 316 2 = 155 2 + 197 2 – 2(155)(197) = cos B – 0.6062 cos B B cos –1 (– 0.6062) 127.3° Use inverse cosine.