Independence of Variables

I have this problem that I am stuck on and I know I must be looking at it wrong but if someone could help me that would be great. are X1, and X2 independent?
p(x1,x2)= { 2/(n(n+1)), for x1= 1,2,...n and x2=1.2,...x1
0, otherwise

In order to find independence I just apply p(x1,x2)= px1(x1)px2(x2) for all x,y.
For some reason I am having trouble proving independence due to not being able to solve each summation. WHAT AM I DOING WRONG????