a ring shape metal plate has an internal radius of 7 cm and an external radius of 10.5 cm . if the cost of material is rupees 5 per sq.cm , find the cost of 25 rings .
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area of ring=area of outer plate -area of inner plate
let r is the inner plate radius
R is outer plate radius
then ,
area of ring =pi.R^2 - pi.r^2
=pi (R^2 -r^2 )
=pi (R - r)(R + r)
now put r =7 cm and R =10.5 cm
area of ring =pi. (10.5-7)(10.5+7)
=22/7 x (3.5)(17.5)
=192.5 cm^2
now ,
cost of one ring =192.5 x 5 Rs
hence ,
cost of 25 rings =192.5 x 125 Rs
=24062.5 Rs
let r is the inner plate radius
R is outer plate radius
then ,
area of ring =pi.R^2 - pi.r^2
=pi (R^2 -r^2 )
=pi (R - r)(R + r)
now put r =7 cm and R =10.5 cm
area of ring =pi. (10.5-7)(10.5+7)
=22/7 x (3.5)(17.5)
=192.5 cm^2
now ,
cost of one ring =192.5 x 5 Rs
hence ,
cost of 25 rings =192.5 x 125 Rs
=24062.5 Rs
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