1/n2=(n-1)/[n2(n-1)]
=n/[n2(n-1)]-1/[n2(n-1)]
=1/[n (n-1)] -1/[n2(n-1)]
=1/(n-1) -1/n-1/[n2(n-1)]
当n>2时,总有1/[n2(n-1)]>0
1/12+1/22+1/32+…+1/20002
=1/12+1/(2-1) -1/2-1/22(2-1)
+1/(3-1) -1/3-1/32(3-1)+…
+1/(2000-1) -1/2000-1/20002(2000-1)
=1/12+1/(2-1)-1/22(2-1) -1/32(3-1)-…
-1/2000-1/20002(2000-1)
=2-1/22(2-1) -1/32(3-1)-…-1/20002(2000-1) -1/2000<2
证毕
是1的平方,2的平方,3的平方,2000的平方