计算1/x×(x+3)+1/(x+3)×(x+6)+1/(x+6)×(x+9)+1/(x+9)×(x+12)
问题描述：
计算1/x×(x+3)+1/(x+3)×(x+6)+1/(x+6)×(x+9)+1/(x+9)×(x+12)
答
1/x×(x+3)=（1/x-1/(x+3)）/3
1/(x+3)×(x+6)=1/（1+3）x-1/(x+6)）/3
……
以此类推,原式=（1-1/(x+12)）/3=(x+11)/(3x+36)