8 Let $M=\{1,2, \cdots, 9\}$, and take the product of every two distinct numbers in it. Then the sum of all these products is $\qquad$ .
(8) 870 Hint: The required sum is
$$
\frac{1}{2}\left[(1+2+\cdots+9)^{2}-\left(1^{2}+2^{2}+\cdots+9^{2}\right)\right]=870 .
$$