The line segment connecting the center of the circle and any point on the circle is called radius, and the letter is R (radius).
The line segment whose two ends pass through the center of the circle is called diameter, and the letter is expressed as D (diameter). The straight line with the diameter is the symmetry axis of the circle.
A line segment connecting any two points on a circle is called a chord. The longest chord is the diameter.
The part between any two points on a circle is called arc for short. An arc larger than a semicircle is called an optimal arc and is represented by three letters. An arc smaller than a semicircle is called the lower arc and is represented by two letters. A semicircle is neither an optimal arc nor a suboptimal arc. The optimal arc is greater than 180 degrees, and the suboptimal arc is less than 180 degrees.
The circumference formula of a circle =C=πd=2πr≈6.28r[ 1]
The area formula of a circle =S=π×r×r[2]
(By analogy, the formula of the circumference of a semicircle =C/2=πr≈3. 14r area =S/2=π×r×r÷2).
A figure surrounded by two radii and an arc is called a sector.
A figure surrounded by a chord and an arc opposite it is called an arch.
The angle of the vertex on the center of the circle is called the central angle.
The angle at which the vertex is on the circumference and both sides intersect with the circle is called the circumferential angle.
The ratio of the circumference of 10 to the diameter and length of a circle is called pi. It is a transcendental number, usually expressed by π = 3. 14 159265 ... In practical application, π≈3. 14.
The fillet of 1 1 is equal to half the central angle of the same arc.