Ex.13.3 Q1 Surface Areas and Volumes Solution - NCERT Maths Class 9

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Diameter of the base of a cone is \(10.5\,\rm cm\) and its slant height is \(10\,\rm cm.\) Find its curved surface area.

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Ex exercise-13-3 | Question 1

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Curved surface area of a right circular cone of base radius r and slant height l is \(\begin{align}\pi rl. \end{align}\)

What is known?

Diameter of the base and slant height.

What is unknown?

Curved surface area.


Diameter \(= 2r = 10.5{\rm\, {cm}}\)

\(\begin{align}\therefore r = \frac{{10.5}}{2}cm \end{align}\)

Slant height \(\begin{align}l = 10\,\,\,cm \end{align}\)

Curved surface area\(\begin{align} = \pi rl \end{align}\)

\[\begin{align} &= \frac{{22}}{7} \times \frac{{10.5}}{2} \times 10\\ &= 165\,\,c{m^2} \end{align}\]


Curved surface area of the cone \(\begin{align} = 165\,\,c{m^2} \end{align}\)