一、常用分数、小数的互化

1/2 =0.5=50%

1/3 ≈0.333 =33.3%

2/3 ≈0.667=66.7%

1/4 =0.25=25%

3/4 =0.75=75%

1/5 =0.2=20%

2/5 =0.4=40%

3/5 =0.6=60%

4/5 =0.8=80%

1/6 ≈0.167=16.7%

5/6 ≈ 0.833=83.3%

1/8 =0.125=12.5%

3/8 =0.375=37.5%

5/8 =0.625=62.5%

7/8 =0.875=87.5%

1/9 ≈ 0.111=11.1%

1/10 =0.1=10%

1/20=0.05=5%

3/20=0.15=15%

7/20=0.35=35%

9/20=0.45=45%

11/20=0.55=55%

13/20=0.65 =65%

17/20=0.85=85%

19/20=0.95=95%

1/16 =0.0625=6.25%

1/32 =0.03125=3.125%

1/64 =0.015625=1.5625%

1/7 =0.142857142857…≈0.143 =14.3%

2/7 =0.285714285714…≈0.286=28.6%

3/7 =0.428571428571…≈0.429=42.9%

4/7 =0.57142857142…≈0.571=57.1%

5/7 =0.714285714285…≈0.714=71.4%

6/7 =0.857142857142…≈0.857=85.7%

熟记：142857

二、常用圆周率的计算

3.14×1=3.14

3.14×2=6.28

3.14×3=9.42

3.14×4=12.56

3.14×5=15.70

3.14×6=18.84

3.14×7=21.98

3.14×8=25.12

3.14×9=28.26

3.14×10=31.40

3.14×11=34.54

3.14×12=37.68

3.14×16=50.24

3.14×18=56.52

3.14×20=62.80

3.14×25=78.50

3.14×32=100.48

3.14×36=113.04

3.14×49=153.86

3.14×64=200.96

3.14×81=254.34

3.14×121=379.94

三、常用的完全平方数

1²=1

2²=4

3²=9

4²=16

5²=25

6²=36

7²=49

8²=64

9²=81

10²=100

11²=121

12²=144

13²=169

14²=196

15²=225

16²=256

17²=289

18²=324

19²=361

20²=400