原式=⅓{[1/x-1/(x+3)]+[1/(x+3)-1/(x+6)]+[1/(x+6)-1/(x+9)]+...+[1/(x+99)-1/(x+102)] =⅓{1/x-1/(x+102)} =⅓[102/x(x+102)]=34/[x(x+102)]
计算如下：
