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That is the definition. If you take your unit circle (a circle with radius 1 centered at the origin (0,0). you start at (1,0) and go counterclockwise around the circle 90° you end up at (0,1) that 0 is the cosine of the angle 90°

In fact, you don't even need the unit circle. Take a circle of any radius r, and draw a ray at 90 degrees. This will intersect the y-axis. So as above, the coordinates are (0,r) (instead of (0,1)) so cos(90 degrees)=x/r=0/r=0

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Q: Why is the cosine of 90 equal to 0?
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