sn=1+2+3+……+(2n-1)+2n
2sn=[1+2+3+……+(2n-1)+2n]+[1+2+3+……+(2n-1)+2n]
=[1+2n]+[2+(2n-1)]+[3+(2n-2)]+……+[(n-1)+(n+2)]+[n+(n+1)]+……+[(2n-1)+2]+[2n+1]
=[1+2n](2n)
sn=(1+2n)n