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unit circle

The circle of radius 1 centered at the origin, x^2+y^2=1.

The unit circle is the circle of radius 1 centered at the origin, x^2+y^2=1. For an angle \theta in standard position, the point where its terminal side crosses the unit circle has coordinates (\cos\theta,\sin\theta) — this defines sine and cosine for every real angle, not just acute ones.

See also

terminal side cofunction
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