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Q : 2     The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in Fig \small 13.32. Eight such spheres are used for this purpose, and are to be painted silver. Each support is a cylinder of radius \small 1.5\hspace{1mm}cm and height 7 cm and is to be painted black. Find the cost of paint required if silver paint costs 25 paise per \small cm^2 and black paint costs 5 paise per \small cm^2.
 

            

Given, The radius of the wooden spheres =   The surface area of a single sphere =  Again, the Radius of the cylinder support =  Height of the support =   The base area of the cylinder =  Now, Cost of painting silver =   Cost of painting 1 wooden sphere = Cost of painting silver =  Now, Curved surface area of the cylindrical support =  Now, Cost of painting  black =   Cost of painting 1 such...

Q : 3    The diameter of a sphere is decreased by \small 25\%. By what per cent does its curved surface area decrease?
 

Let the radius of the sphere be  Diameter of the sphere =  According to question, Diameter is decreased by  So, the new diameter =  So, the new radius =  New surface area =  Decrease in surface area =   Percentage decrease in the surface area = 

Q : 1    A wooden bookshelf has external dimensions as follows: Height \small =110\hspace{1mm}cm, Depth \small =25\hspace{1mm}cm, Breadth \small =85\hspace{1mm}cm (see Fig. \small 13.31). The thickness of the plank is \small 5\hspace{1mm}cm everywhere. The external faces are to be polished and the inner faces are to be painted. If the rate of polishing is \small 20\hspace{1mm} paise per \small cm^2 and the rate of painting is \small 10\hspace{1mm} paise per \small cm^2, find the total expenses required for polishing and painting the surface of the bookshelf.

            

External dimension od the book shelf = (Note: There is no front face) The external surface area of the shelf = We know, each stripe on the front surface is also to be polished. which is 5 cm stretch. Area of front face = Area to be polished = Cost of polishing  area = Cost of polishing  area =  Now, Dimension of inner part =  Area to be painted in 3 rows = Cost of painting  area =...
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