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Question

# The front compound wall of a house is decorated by wooden spheres of a diameter 21 cm, placed on small supports as shown in the figure. Eight such spheres are used for this purpose and are to be painted silver. Each support is a cylinder of radius 1.5 cm and height 7 cm and is to be painted black. Find the cost of paint required if silver paint costs 25 paise per cm2.

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Solution

## r=212=10.5cm, SA=4πr2=4×227×(10.5)2=1386cm2Cylindrical support,r1 of circular end=1.5cm, h=7cmCurved Surface Area =2πrh=2×227×1.5×7=66cm2Area of circular end =πr2=227×(1.5)2=7.07 cm2Area to be painted silver =8×(1386−7.07)cm2=8×1378.9=11031.44cm2Cost of silver color =Rs.0.25×11031.4=Rs.2757.8Area to be painted black =8×66=578cm2Cost of painting black color=1528×0.05=Rs.26.40Total cost =Rs.2757.8+26.4=Rs.2784.2

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