A full circle is 360 degrees of arc. Half is 180. A quarter is 90. Circle area and circumference are a different job, in circumference and area.
Slice from 360
A central angle of 40° is 40/360 of the circle. Arc length uses that fraction, in arc length. A straight line is 180°, in a straight angle is 180.
The short list is in ACT Math formulas.
The bottom line
Full turn: 360°. Fractions of a circle start from that.
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