cos pi= -1
using the 360 circle ir those quadrants
i forgot what it was, link
First of all "pi" is NOT "90". Get that out of your head! Nor does "pi radians" correspond to "90 degrees" if that was what you meant. "pi radians" corresponds to "180 degrees", half a circle. The crucial thing you need to remember is that a full circle is 360 degrees, 2pi radians. Perhaps it would help to remember that a circle has circumference pi*d= 2pi *r. That's where the "2pi" comes from.
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