The unit circle
A circle of radius 1 centered at the origin (unit circle) is the main model of grade 99 trigonometry. The endpoint of the rotation is a point whose coordinates come from the angle θ\theta. On this model coordinates are measured in units of length from the center:
cos⁡θ\cos\theta is the xx-coordinate; sin⁡θ\sin\theta is the yy-coordinate
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