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Standard IX

Mathematics

Curved surface area of a Right Circular cone

The total sur...

Question

The total surface area of a cone whose radius is $\frac{r}{2}$ and slant height 2l is
(a) $2 π r (l + r)$
(b) $π r (l + \frac{r}{4})$
(c) $πr (l + r)$
(d) $2 π r l$

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Solution

(b) $π r (l + \frac{r}{4})$

$Total surface area of the given cone = π (\frac{r}{2}) \times 2 l + π {(\frac{r}{2})}^{2} = π (r l + \frac{r^{2}}{4}) = π r (l + \frac{r}{4})$

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The total surface area of a cone whose radius is r2 and slant height 2l is (a) 2πr(l + r) (b) πrl+r4 (c)πr(l + r) (d) 2πrl

(b) πrl+r4 Total surface area of the given cone =πr2×2l +πr22 =πrl+r24 =πrl+r4

The total surface area of a cone whose radius is $\frac{r}{2}$ and slant height 2l is
(a) $2 π r (l + r)$
(b) $π r (l + \frac{r}{4})$
(c) $πr (l + r)$
(d) $2 π r l$

(b) $π r (l + \frac{r}{4})$

$Total surface area of the given cone = π (\frac{r}{2}) \times 2 l + π {(\frac{r}{2})}^{2} = π (r l + \frac{r^{2}}{4}) = π r (l + \frac{r}{4})$