3.14是π的近似值，熟記幾個πr²的值有利於我們方便計算。

1²×3.14 =3.14

2²×3.14 =12.56

3²×3.14 =28.26

4²×3.14 =50.24

5²×3.14= 78.5

6²×3.14 =113.04

7²×3.14= 153.86

8²×3.14 =200.96

9²×3.14 =254.34

10²×3.14 =314

11²×3.14 =379.94

12²×3.14 =452.16

13²×3.14 =530.66

14²×3.14=615.44

15²×3.14=706.5

16²×3.14=803.84

17²×3.14=907.46

18²×3.14=1017.36

19²×3.14=1133.54

20²×3.14=1256

21²×3.14=1384.74

22²×3.14=1519.76

23²×3.14=1661.06

24²×3.14=1808.64

25²×3.14=1962.5

26²×3.14=2122.64

27²×3.14=2289.06

28²×3.14=2461.76

29²×3.14=2640.74

30²×3.14 =2826

31²×3.14=3017.54

32²×3.14=3215.36

33²×3.14 =3419.46

34²×3.14 =3629.84

35²×3.14 =3846.5

36²×3.14 =4069.44

37²×3.14=4298.66

38²×3.14=4534.16

39²×3.14=4775.94

40²×3.14=5024

41²×3.14=5278.34

42²×3.14 =5538.96

43²×3.14=5805.86

44²×3.14=6079.04

45²×3.14=6358.5

46²×3.14 =6644.24

47²×3.14 =6936.26

48²×3.14=7234.56

49²×3.14 =7539.1