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# Ex.13.7 Q3 Surface Areas and Volumes Solution - NCERT Maths Class 9

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## Question

The height of a cone is $$15\rm\, cm.$$ If its volume is \begin{align}1570\,\,c{m^3} \end{align}, find the radius of the base.(Use $$p = 3.14$$).

Video Solution
Surface-Areas-And-Volumes
Ex exercise-13-7 | Question 3

## Text Solution

Reasoning:

Volume of the cone is \begin{align} \frac{1}{3} \end{align} times of the volume of a cylinder having radius r and height h i.e. \begin{align} = \frac{1}{3}\pi {r^2}h \end{align}

What is  known?

Volume of the cone and the height of the cone.

What is  unknown?

Steps:

Height $$(h) = 15\,\rm cm$$

Radius $$(r) = ?$$

Volume of cone

\begin{align}&= \frac{1}{3}\pi {r^2}h \\1570&= \frac{1}{3}\pi {r^2}h \\ 1570 &= \frac{1}{3} \times 3.14 \times {r^2} \times 15 \\{r^2} &= \frac{{1570 \times 3}}{{15 \times 3.14}} = \frac{{4710}}{{47.1}} \\{r^2} &= 100 \\ r &= \sqrt {100} = 10\,\,\rm\,cm \end{align}

Radius of the base $$= 10 \rm\,cm$$