1又3/1-12/7+20/9-30/11+42/13-56/15

原式=1+1/3-(3+4)/12+(4+5)/20-(5+6)/30+(6+7)/42-(7+8)/56
=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8
=1-1/8
=7/8
备注：每个分数的分子是两个数相加,分母是两个数相乘.