Question

The diameter of sphere is decreased by 20%. By what per cent does its CSA decreases or increases?

Answer 1

Let the diameter be 10 units
radius = 5 units
CSA = 4πr² = 4×π×25 = 100π sq units
now, diameter 20% decreased = 8 units
radius = 4 units
CSA' = 4πr'² = 4π× 16 = 64π sq units
difference = 100π - 64π =36π
Percentage decrease = (36π/100π)×100 = 36%