a = 2pi*r*h
a' = 2pi(2/3 r)(4/7 h) = 2pi*rh*8/21
a:a' = 1:8/21 = 21:8
Note that the ratio is just the product of the linear ratios.
The radii of two cylinders 3:2 and height 7:4.find the ratios of their curved surface area.
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