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