1+1/2^2;+1/3^2;+1/4^2+1/n^2 < 1+1/（2*1),+1/(3*2),+1/(4*3),+.+1/[n*(n-1)]=1+[(1-1/2)+(1/2-1/3)+(1/3-1/4)+.+(1/n-1-1/n)]=1+1-1/n=2-1/n< 2 .