计算2/1+6/1+12/1……+90/1
问题描述：
计算2/1+6/1+12/1……+90/1
答
1/2=1-1/2
1/6=1/2-1/3
1/12=1/3-1/4
…
2/1+6/1+12/1……+90/1
=1-1/2+1/2-1/3....+(1/9-1/10)
=1-1/10
=0.9
答
2/1+6/1+12/1……+90/1
=2+3+4……+90
=（2+90）89/2
=4094
答
PS:1/6=1/2-1/3
1/12=1/3-1/4
所以1/2+(1/2-1/3)+(1/3-1/4)+.+(1/9-1/10)=1/2-1/10=2/5=0.4