计算1/x(x+3)+1/(x+3)(x+6)+1/(x+6)(x+9)
问题描述：
计算1/x(x+3)+1/(x+3)(x+6)+1/(x+6)(x+9)
rt
答
1/x(x+3)+1/(x+3)(x+6)+1/(x+6)(x+9)=1/3[3/x(x+3)+3/(x+3)(x+6)+3/(x+6)(x+9)]=1/3[1/x-1/(x+3)+1/(x+3）-1/(x+6)+1/(x+6）-1/(x+9)]=1/3[1/x-1/(x+9)]=1/3*(x+9-x)/x(x+9)=1/3*9/x(x+9)=3/x(x+9)