In triangle ABC, b = 5 cm, c = 8 cm and the angle between them is A = 60°. The length of side a is:

The cosine rule gives $a^2=b^2+c^2-2bc$·cosA = $25+64-2(5)(8)(1/2)=89-40=49$, so $a=7$ cm. Dropping the $-2bc$·cosA term altogether is Pythagoras and gives √89 ≈ 9.43; taking cos60° with the wrong sign gives √129 ≈ 11.36.