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In the image, P is center point of the circle, line C is a chord and line S is a secant.
If two chords AB, CD intersect at P (not necessarily the center), then AP * PB = CP * PD
All inscribed angles going to the two same points on the circle (but from different vertices) have the same angle angle of intersecting secants theorem:
angle formed by the secant intersection = (opposite arc - adjacent arc)/2
An angle outside the circle with two secants (or tangents) will have an angle that is 1/2 * (difference of the intercepted arcs)
coterminal angle - the rest of the circle outside the angle.
eg. angle of 30°, the coterminal is 330° in radians, use the absolute value. 2π - |angle|, or |angle| - 2π for the negative angle
area of an arc (from a central angle):
a = rad*r²/2 .... comes from a = πr² for full circle, and the proportion of a circle in the arc is rad/2π so a = (rad/2π)*(πr²)