7. The function $f$ defined on the set of positive integers $\mathbf{N}^{-}$ satisfies: $f(1)=1$, and for any natural numbers $m, n$, $f(m)+$ $f(n)=f(m+n)-m n$, then $f(m)=$ $\qquad$ .
7. $\frac{m(m+1)}{2}$ Let $n=1, f(m+1)-f(m)=m+1$, then use partial fraction decomposition.