1/2+1/6+1/12+1/20=

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1/2+1/6+1/12+1/20=

1/2+1/6+1/12+1/20=
1/2+1/6+1/12+1/20=

1/2+1/6+1/12+1/20=
4/5

1/2+1/6+1/12+1/20
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5
=1-1/5=4/5

(30+10+5+3)/60=4/5

1/2+1/6+1/12+1/20=
1/1*2+1/2*3+1/3*4+1/4*5
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5
=1-1/5
=4/5
希望你能看懂！

7/15

1/2+1/6+1/12+1/20
=（1-1/2）+（1/2-1/3）+（1/3-1/4）+（1/4-1/5）
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5
=1-1/5
=4/5

(30+10+5+3)/60=4/5 =0.8

4/5
1-1/5=4/5