拆分法：形如1/(1×2)=1/1-1/2
所以原式 = 1/(1×2)＋1/(2*3)+1/(3*4)+……＋1/(9*10)
=1/1-1/2+1/2-1/3+1/3-1/4+……＋1/9-1/10
= 1-1/10
= 9/10