一个5年分数计算

分数写反了是吧

1/2=1-1/2
1/6=1/2-1/3
1/12=1/3-1/4
……
1/90=1/9-1/10

结果=1-1/10=9/10=0.9

2/1+6/1+12/1+20/1+30/1+42/1+56/1+72/1+90/1
=1/1*2+1/2*3+1/3*4+....+1/9*10
=1-1/2+1/2-1/3+.....+1/9-1/10
=1-1/10
=9/10 (十分之九)

1/2+1/16+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1-1/2+1/2-1/3+.....+1/9-1/10
=1-1/10
=0.9