Pn=1/a1a2 +1/a2a3+.+1/(an)(an+1)=1/a1-1/a2+1/a2-1/a3+.+1/(an)-1/(an+1) =1/a1-1/(an+1) Sn=n^2,如果是等差数列,an=1+2（n-1）=2n-1 limPn=1-lim1/[2n-1]=1
