1/(2n-1)(2n+1)=1/2*[1/(2n-1)-1/(2n+1)]sn=1/2*(1-1/3)+1/2*(1/3-1/5)+....+1/2*[1/(2n-1)-1/(2n+1)]=1/2*[1-1/3+1/3-1/5+............+1/(2n-1)-1/(2n+1)]=1/2*[1-1/(2n+1)]=1/2*2n/(2n+1)=n/(2n+1)
