1. Know the names of the circle and its parts: diameter d, radius r, center o, perimeter c and area s, and know that the circle has countless diameters and radii, and their lengths are equal. The same is true of semicircles.

2. Both a circle and a semicircle are axisymmetric figures. A circle has countless symmetry axes, while a semicircle has only one. Note that the axis of symmetry is not equal to the diameter, because the axis of symmetry can extend indefinitely.

[13] C circle formula: (pai is not easy to play, which is represented by X below) xd.

S circle formula: x times the square of r

C semicircle formula: χ χ⒉57d or χ χ14r.

S semicircle formula: x times half r square

4. You can calculate the area of the combined graph (forget it, the area is calculated by adding one and subtracting one).