题目应该是
1/2*5+1/5*8+1/8*11+...+1/98*101
吧
1/2*5=(1/2-1/5)/3
1/5*8=(1/5-1/8)/3
1/8*11=(1/8-1/11)/3
.
.
.
1/98*101=(1/98-1/101)/3
把上式累加得到
1/2*5+1/5*8+1/8*11+...+1/98*101
=(1/2-1/101)/3
=33/202