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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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