Circular segmentCreative Commons Attribution/Share-Alike License 3.0 (Unported)
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But if you don't know radius and angle you still can caluclate the segment parameters by chord length and segment height:

Segment defined by chord and heightCreative Commons Attribution/Share-Alike License 3.0 (Unported)
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Formula for segment radius by chord and height:
R=\frac{h}{2}+\frac{c^2}{8h}

Then, you can caluclate segment angle using the following formula:
\alpha=2\arcsin{ \frac{c}{2R} }

You may also use the following calculator to obtain segment area by its radius and height:

Area of circle segment by radius and heightCreative Commons Attribution/Share-Alike License 3.0 (Unported)
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