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$AOB$ is a sector of a circle with center $ O$ and radius $OA = 10$.
Circle with radius $3$ is inscribed in this sector such that it touches radius $OA$, radius $OB$ and arc $AB$.
Find the length of the chord $AB$.

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I don't know where to begin. To calculate the length of $AB$ ,
we'll need the length of perpendicular from $O$ to $AB$.
( then we can use pythagoras theorem to get half of $AB$ and then $AB$) .
But how can I find that?

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The distance from the centre of the inscribed circle to point $O$ is $7$, so the angle between $AO$ and the perpendicular line from $O$ to $AB$ is $\theta=\arcsin\left(\frac{3}{7}\right)$, assuming that lines $OA$ and $OB$ are tangents to the inscribed circle.

Thus the length of $AB$ is

$$AB=2\times OA\times\sin(\theta)=2\times10\times\frac{3}{7}=8\frac{4}{7}$$

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  • $\begingroup$ Wow , very good solution! I never thought to use arcsin. $\endgroup$ – A Googler Apr 4 '14 at 17:54
  • $\begingroup$ Glad you like the solution. The arcsin is useful when you can find a right angled triangle with two sides you can easily calculate - which was the triangle formed by the centre of the inscribed circle, point $O$ and the point where $OA$ touches the inscribed circle. Once you have found the angle, you can easily work out $AB$. $\endgroup$ – Alijah Ahmed Apr 4 '14 at 18:01
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Suppose that the center of the smaller circle is $O'$. Since $OA, OB$ are tangents to the smaller circle, then $O'O$ bisects $\angle AOB$. Let $\angle O'OB = \theta$, then $\sin \theta = \frac {3} {7} $, we can find $\cos 2\theta$ by the formula $\cos 2\theta = 1-2\sin^2 \theta$ and we get that $\cos 2\theta = \frac {31} {49}$. Now by Cosine Law we get: $$OA^2 +OB^2 -2\cdot OB\cdot OA\cdot \cos 2\theta =AB^2 $$ $$\Rightarrow 10^2 +10^2 -2\cdot 10^2\cdot \frac {31} {49} =AB^2 $$ $$\Rightarrow 200(1-\frac {31} {49})=AB^2 $$ $$AB=\frac {60} {7}=8\frac {4}{7}$$

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