Each side of a rhombus is 25 cm and one of its diagonals is 30 cm long. Find the area of the rhombus in ${\text{cm}}^2$.

The diagonals bisect each other at right angles, so one half-diagonal is 15 cm and the other = √$(25^2-15^2)=$ √400 = 20 cm. The second diagonal is therefore $2\times 20=40$ cm. Area = ½ × d₁ × d₂ = ½ × $30\times 40=600{\text{cm}}^2$.