Question

Find $r$, if $V=22,R=2,l=4$ and $\pi =3\frac{1}{7}$.

• A. $2$
• B. $3$
• C. $1.5$
• D. $2.5$

Answer 1

The correct option is C $1.5$

Given, $V=\pi (R^2-r^2)l$

$\Rightarrow \frac{V}{\pi l}=R^2-r^2$

$\Rightarrow r^2=R^2-\frac{V}{\pi l}$

$\Rightarrow r^2=\frac{\pi R^{2}l-V}{\pi l}$

$\Rightarrow r=\sqrt{\frac{\pi R^{2}l-V}{\pi l}}$
When, $V=22,R=2,l=4,\pi =3\frac{1}{7}=\frac{22}{7}$,
we get $r=\sqrt{\frac{\frac{22}{7}\times 2^2\times 4-22}{\frac{22}{7}\times 4}}=\sqrt{\frac{4\times 4-7}{4}}=\sqrt{\frac{9}{4}}=\frac{3}{2}=1.5$