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The Unit Circle is a circle that has a radius of 1 and a center at the origin.

If you look at the unit circle 90 degrees is at the point (0,1).

Cosine is equal to the x value of a point on the Unit Circle.

The line created to the point (0,1) on the unit circle when the degree is 90 is completely vertical, which in turn makes the x value 0 and thus, cosine of 90 = 0.

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