Question

2/1*6/1*12/1*20/1*.........前30位数的积，100位数相乘的积，90位数相乘的积。。

写算式，过程，写理由，要清楚。

Answer 1

=1-1/2+1/2-1/3+1/3-1/4+。。。+1/30-1/31
=1-1/31
=30/31

100位数相乘的积
=1-1/2+1/2-1/3+1/3-1/4+。。。+1/100-1/101
=1-1/101
=100/101

90位数相乘的积
=1-1/2+1/2-1/3+1/3-1/4+。。。+1/90-1/91
=1-1/91
=90/91