Yeah, back when I was first introduced to calculus and integration, I was surprised to see ∫4pir^2 dr = 4pi ∫r^2 dr = 4pi*r^3/3. Then I learnt about the Cavalie

Yeah, back when I was first introduced to calculus and integration, I was surprised to see ∫4pir^2 dr = 4pi ∫r^2 dr = 4pi*r^3/3. Then I learnt about the Cavalieri's principle and it comes clear in mind.