# How to calculate the coordinates of the middle point of a given arc?

Does anybody know how to solve this problem? I am trying to calculate the green sides of this triangle:

I know / have :

the arc length, the arch base, the radius, and the h (distance from the red dot to the center of the circle)

Is it possible :) ?

Thanks a lot!

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duplicated here: math.stackexchange.com/questions/91769 –  Zev Chonoles Feb 17 '12 at 17:06

Yes. In that case, it would be easiest to translate your coordinate system such that the origin lies at the centre of the circle (i.e. subtract the radius from all coordinates); then you could calculate the angle at which that black dot lies from the centre as the arctangent of the ratio of its coordinates, and then add the arctangent of the ratio of $dB$ to $h$ to get the angle $\alpha$ at which the red dot lies from the centre; then its coordinates (in the translated coordinate system) are $(h\cos\alpha,h\sin\alpha)$. –  joriki Dec 15 '11 at 12:23