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## Homework Statement

http://tng.trekcore.com/1.JPG [Broken]

I'm trying to prove that the circle is symmetrical by showing that x² + y² = a² holds when the circle rotates.

I know that this is proved given the following two formulae:

x = x'cosθ - y'sinθ

y = x'sinθ + y'cosθ

but I don't know where those two equations have come from based on my diagram. Help!

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