JAMB 1990 · UME · Q35

Find the length of a side of a rhombus whose diagonals are 6 cm and 8 cm.

Worked solution (try it first)

The diagonals of a rhombus bisect each other at right angles, so each side is the hypotenuse of a right-angled triangle with legs $6 \div 2 = 3$ cm and $8 \div 2 = 4$ cm.
By Pythagoras, the side is $\sqrt{3^2 + 4^2} = \sqrt{25} = 5$ cm, option B.