The content of the second volume of grade five is to guide students to explore and master the area formula of a circle on the basis that students have mastered the basic characteristics of the circle and the formula of its circumference. Through the teaching of three examples, the area of a circle is deduced by two different strategies, so that students can fully feel the rationality of the derivation process of the formula of the area of a circle.

Formula related to circle:

1, circular area: S=πr? ，S=π(d/2)？ . (d is the diameter and r is the radius).

2. area of semicircle: s semicircle = (π r 2)/2. (r is the radius).

3. Ring area: s great circle -S small circle = π (r 2-r 2) (r is the radius of great circle and r is the radius of small circle).

4. Circumference: C=2πr or c = π d..(d is the diameter and r is the radius).

5. The circumference of a semicircle: d+(πd)/2 or d+π r (d is the diameter and r is the radius).

6. The area of the circle where the sector is located is divided by 360 and multiplied by the angle n of the central angle of the sector, as shown below:

S=n/360×πr？ .

S=πr？ ×L/2πr=Lr/2(L is arc length and r is sector radius).