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 $=\frac{7}{2} \times 2=7$
And length $=22-\frac{7}{2}=\frac{37}{2}=18.5\ m$
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 $=Rs. 4 \text{per} \ {m}^3$
Total expenditure $=Rs. 4 \times 223.125= Rs. 892.50$

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