Probability

The p.m.f. of a random variable \(\mathrm{X}\) is $$\mathrm{P}(x)=\left\{\begin{array}{cl}\frac{2 x}{\mathrm{n}(\mathrm{n}+1)}, & x=1,2,3, \ldots \mathrm{n} \\ 0, & \text { otherwise }\end{array}\right.$$, then \(\mathrm{E}(\mathrm{X})\) is