8. In the arithmetic sequence $\left\{a_{n}\right\}$, if $a_{m}=n^{2}, a_{n}=m^{2}$, then $a_{m+n}=$

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8. $-m n$.

Since $a_{m}=a_{m+n}-n d=n^{2}, a_{n}=a_{m+n}-m d=m^{2}$, we have $m a_{m}-n a_{n}=(m-n) a_{m+n}=m n^{2}-$ $n m^{2}$, thus $a_{m+n}=-m n$.