Divide the circle into several parts evenly and you can make an approximate rectangle. The width of the rectangle is equal to the radius (r) of the circle, and the length of the rectangle is half the circumference (c) of the circle. The area of the rectangle is ab, and the area of the circle is: the radius (r) of the circle times the semi-circumference c, and S=r*C/2=r*πr, and the relevant formula is as follows:
1, circular area = pi × radius× radius.
2. area of semicircle: s semicircle =(πr2)÷2
3. Area of semicircle = π× radius× radius ÷2
4. Area of the circular ring: s Great Circle -s Small Circle =π(R2-r2)(R is the radius of the great circle and R is the radius of the small circle).
5. Annular area = outer large circle area-inner small circle area
6, the circumference of the circle = diameter × π
7, semicircle circumference = π× radius+diameter
Extended data:
Formula derivation: perimeter formula
Perimeter (c): the diameter (d) of a circle. If the circumference (c) of a circle divided by the diameter (d) of the circle equals π, then the meaning of multiplication equals π times the diameter (d) of the circle, and c = π d. The diameter (d) of the same circle is twice the radius (r) of the circle, so the circumference (c) of the circle equals 2 times π times the radius (r) of the circle, and c = 2π r.