A circle’s radius (r) is the distance between the center of the circle and any point on its perimeter.
The diameter (d) of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle.
It is 2 x the radius of the circle.
The circumference (C) of a circle is the distance around the outside of the circle and is expressed as C = πd.
When a line is tangent to a circle at a point, it means that the line and the circle’s radius drawn to a point on the line form a 90° angle.
The central angle is the angle formed between the two radii that contact two points of tangency.
Arc length is the distance between two points along a section of a curve.
A circle is 360° all the way around; therefore, if you divide an arc’s central angle by 360°, you can find the fraction of the circle’s circumference made up by the arc.
Then, if you multiply the length all the way around the circle (the circle’s circumference) by that fraction, you get the length along the arc - ie the arc length!
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