做这种类型的题目要利用公式计算.
记住公式：1/(1*2*3)+1/(2*3*4)+……+1/(n*(n+1)*(n+2)=1/2*[1/(1*2)-1/n*(n+1)*(n+2)]
1x2x3分之1+2x3x4分之1+……+10x11x12分之1
=1/(1*2*3)+1/(2*3*4)+……+1/(10*11*12)
=1/2*[1/2-1/(10*11*12)]
=1/2*[1/2-1/1320]
=1/2*659/1320
=659/2640 .