Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 3 - No. 41)

In triangle PQR, p = 5 cm, q = 7 cm, and angle R = 60°. What is the approximate length of side r?

6.2 cm

6.5 cm

7.1 cm

8.0 cm

Using the Law of Cosines, r² = p² + q² - 2pq*cos(R), r² = 5² + 7² - 2*5*7*cos(60°) = 25 + 49 - 35 = 39. r = √39 ≈ 6.2.