7. Given sets of real numbers $A, B$, define the operation $A \otimes B=\{x \mid x=a b+a+b, a \in A, b \in B\}$. Let $A=\{0,2,4, \cdots, 18\}, B=\{98,99,100\}$, then the sum of all elements in $A \otimes B$ is
Solve:
$x=a b+a+b=(a+1)(b+1)-1$, then the sum of all elements in $A \otimes B$ $=1.99-1+1 \cdot 100-$
$$
\begin{array}{l}
1+1 \cdot 101-1+3 \cdot 99-1+3 \cdot 100-1+3 \cdot 101-1+\cdots+19 \cdot 99-1+19 \cdot 100-1+19 \cdot 101-1 \\
=(1+3+\cdots+19)(99+100+101)-30=100 \cdot 300-30=29970 .
\end{array}
$$