Here the solution says “If AB is the diameter, then C is 90 degrees”. How did they know that? What theorem is it? I looked on prepswift and couldn’t find a video that goes over this.

Hi,

All triangles inscribe in a circle, with one side being the diameter of the circle, have an angle of 90 degrees at the angle opposite the diameter. So they’re all right triangles.

An easy way to understand this is that the diameter forms the diagonal of an inscribed rectangle/square, and you’re looking at half of it.

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