A rectangle has perimeter 56 cm and area 180 cm². What is the length of its diagonal?
Correct: D
Let sides be x,y. Then 2(x+y)=56 → x+y=28, and xy=180. Diagonal d=√(x²+y²)=√[(x+y)²−2xy]=√[28²−2×180]=√[784−360]=√424=√(4×106)=2√106≄20.6 cm. Closest choice is 21 cm.