= Solution
In every derivation beginning at $wA$, the terminal prefix $w$ remains unchanged because no production has a terminal on its left-hand side. By the sentential-form result, each step rewrites the sole variable at the right and appends terminals immediately before its successor variable. Hence
$$
wA\Rightarrow^*vB
$$
forces $v=wu$ for some $u\in\Sigma^*$. Removing the inert prefix $w$ from every sentential form in the derivation gives
$$
\boxed{A\Rightarrow^*uB.}
$$
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