7. Define an operation of sets $A$ and $B$: $A * B=$ $\left\{x \mid x=x_{1}+x_{2}\right.$, where, $\left.x_{1} \in A, x_{2} \in B\right\}$. If $A$ $=\{1,2,3\}, B=\{1,2\}$, then the sum of all elements in $A * B$ is $\qquad$
二、7.14.
$A * B$ 中元素为 $2,3,4,5$, 故其所有元素数字之和为 14 .

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Second, 7.14.
The elements of $A * B$ are $2,3,4,5$, so the sum of all the digits of its elements is 14.