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Rasheed's playing top is shaped like a cone surmounted by a hemisphere. The entire top is $5\,\mathrm{cm}$ high and its diameter is $3.5\,\mathrm{cm}$. Taking $\pi=\frac{22}{7}$, what area has to be coloured? 
A$35.2\,\mathrm{cm}^2$
B$39.6\,\mathrm{cm}^2$
C$42.4\,\mathrm{cm}^2$
D$44.8\,\mathrm{cm}^2$
Answer & Solution
Correct answer: B. $39.6\,\mathrm{cm}^2$
Radius $r=\frac{3.5}{2}=1.75\,\mathrm{cm}$. Height of cone $=5-1.75=3.25\,\mathrm{cm}$, so slant height $l=\sqrt{1.75^2+3.25^2}\approx 3.7\,\mathrm{cm}$. Area to colour $=$ CSA of hemisphere $+$ CSA of cone $=2\pi r^2+\pi rl$. Substituting gives $\frac{22}{7}\cdot\frac{3.5}{2}(3.5+3.7)=39.6\,\mathrm{cm}^2$ approximately.
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