In a right-angled triangle ABC, if angle B = 90°, BC = 3 cm and AC = 5 cm, then the length of side AB is
(a) 3 cm
(b) 4 cm
(c) 5 cm
(d) 6 cm
By Pythagoras, AB² = AC² – BC² = 25 – 9 = 16. So AB = 4 cm.