The internal and external diameters of a hollow hemi-sphereical vessel are 24cm and 25cm respectively. The cost of paint one sq. cm of the surface is 7 paise. Find the total cost to paint the vessel all over. (ignore the area of edge).

Explanation

Total surface area to be painted
=External curved surface + Internal curved surface + Area of ring
='`2πR^2+2πr^2+π(R^2-r^2)`
=`π(2R^2+2r^2+R^2-r^2)`
=`π(3R^2+r^2)`
=`22/7(3x12.52+122)`
=`22/7(468.75+144)`
=`22/7×612.75`
=`1925.78'sq.cm`
Rate of painting =`7` paise per sq.cm
Thus, cost of painting-Area Rate
='`(1925.78×7)/100=Rs134.804`.'