The radius of base of solid cone is 9 cm and its height is 21 cm. It cut into 3 parts by two cuts, which are parallel to its base. The cuts are at height of 7 cm and 14 cm from the base respectively. What is the ratio of curved surface areas of top, middle and bottom parts respectively.
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 1 : 3 : 5
Height of each part = 7 cm
Here,
QV = 3; PV = 
Also,
RU = 6
CSA of upper : CSA of middle : CSA of lower
= π × 3 × : π × 9 × : π × 15 × :
⇒ 3 : 9 : 15 = 1 : 3 : 5
Short Trick:
Sum of all ratios should be a perfect square.
From option 2 = 1 + 3 + 5 = 9 (which is perfect square)
Only option 2 satisfied the condition.