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We know that length of diameter $AB=6$; $CA=2\sqrt{3}$; We should find area of darkened figure I have tried following :

Since $AB$ is diameter  we  can say $ACB$ is right triangle  so  

we can caluclate area of $ABC$, but my question is how calculate area of this triangle inside circle? Please help me.

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    Perhaps you could be able to find $\angle ABC$ using some trinometric functions and then use it to find the angle $\angle AOD$, where D is the intersection of $BC$ with the circle. http://en.wikipedia.org/wiki/Inscribed_angle2011-06-22

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From the metric data you can compute the angle at $B$ since $\tan B = \sqrt 3/3$. Let $D$ be the intersection of $BC$ with the circle. Since the angle at $B$ is known and $OB=OD$, you can easily compute the area of the circular sector $AOD$ and of the triangle $BOD$.

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    The area of $\triangle BOD$ is the same as that of $\triangle AOD$, an equilateral triangle of edge 3.2011-06-22
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$\triangle ABC$ is half an equilateral triangle making $\angle BCA = \pi /3$ and $\angle CBA = \pi /6$, so if $D$ is the intersection of $BC$ and the circle then $\triangle OAD$ is equilateral, and with the circle radius of 3 you can calculate all the areas.

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    thanks a lot i have also read wiki page about inscribed angles everything is clear now for me thanks guys2011-06-22