A cubical block of side 10 cm is surmounted by a hemisphere. What is the largest diameter that the hemisphere can have? Find the cost of painting the total surface area of the solid so formed, at the rate of Rs 5 per 100 sq cm. [Use π = 3.14.]

Length of side of cubical block = a = 10 cm


Since, a cubical block is surmounted by a hemisphere, so, the largest diameter of hemisphere = 10 cm


Since, hemisphere will be touching the sides of cubical block.


Radius of hemisphere = r = Diameter ÷ 2 = 10/2 cm = 5 cm


Surface area of solid = Surface area of cube – Area of circular part of hemisphere + Curved surface area of hemisphere


Total Surface area of solid = 6a2 – πr2 + 2πr2 = 6a2 + πr2


= 6 × 10 × 10 cm2 + 3.14 × 5 × 5 cm2


= 678.5 cm2


Rate of painting = Rs 5/100 cm2


Cost of painting the total surface area of the solid so formed = Total Surface area of solid × Rate of painting


Cost of painting the total surface area of the solid = Rs 5/100 × 678.5


= Rs 33.925


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