The sides of a triangle measure 7 cm, 8 cm and 13 cm. Its largest angle is:

The largest angle faces the longest side, 13 cm. The cosine rule gives cosC = $(49+64-169)/(2\times 7\times 8)=-56/112=-1/2$, and the angle whose cosine is −1/2 is 120°. Losing the minus sign turns the answer into 60°; note also that 49 + 64 ≠ 169, so the triangle is not right-angled.