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

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## Question

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.

Video Solution
Surface-Areas-And-Volumes
Ex exercise-13-3 | Question 1

## Text Solution

Reasoning:

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.

Steps:

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}