Question
In the following diagram a rectangular platform with a semi-circular end on one side is $22$ metreslong from one end to the other end. If the length of the half circumference is $11$ metres. Find thecost of constructing the platform, $1.5$ metres high at the rate of Rs. $4$ per cubic metres.

Answer

Length of the platform $= 22 m$
Circumference of semicircle $= 11 m$
$\therefore$ Radius $=\frac{c \times 2}{2 \times \pi}=\frac{11 \times 7}{22}=\frac{7}{2} m$
Therefore, breadth of the rectangular part $=\frac{7}{2} \times 2=7$
$\text { And length }=22-\frac{7}{2}=\frac{37}{2}=18.5 m$
$\text { Now area of platform }=1 \times b+\frac{1}{2} \pi r^2$
$=18.5 \times 7+\frac{1}{2} \times \frac{22}{7} \times \frac{7}{2} \times \frac{7}{2} m ^2$
$=129.5+\frac{77}{4} m ^2$
$=148.75 m ^2$
Height of the platform $= 1.5 m$
$\therefore$ Volume $=148.75 \times 15=223.125 m ^3$
Rate of construction=Rs 4 perm ${ }^3$
Total expenditure=Rs $4 \times 223.125=$ Rs $892.50$

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