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10-1.Circle and System of Circles
normal
$S_1$ and $S_2$ are two concentric circles of radii $1$ and $2$ respectively. Two parallel tangents to $S_1$ cut off an arc from $S_2$. The length of the arc is
A
$\frac{\pi }{2}$
B
$\frac{2\pi }{3}$
C
$\frac{3\pi }{4}$
D
$\frac{\pi }{4}$
Solution

$\cos \theta=\frac{\mathrm{OM}}{\mathrm{OA}}=\frac{1}{2} \Rightarrow \theta=\frac{\pi}{3}$
$\cos \alpha=\frac{1}{2} \Rightarrow \alpha=\frac{\pi}{3}$
$\operatorname{arc}(\mathrm{AB})=\frac{\pi}{3} \cdot 2 \mathrm{r}$
Standard 11
Mathematics