9.13 How many different sound combinations can be taken from ten selected piano keys, if each sound combination can contain from three to ten sounds?
9.13 For each sound combination, the Slavic keys are pressed simultaneously, so for $k$ sounds we have $C_{10}^{k}$ combinations. Thus, the desired number is $C_{10}^{3}+C_{10}^{4}+\ldots+C_{10}^{10}=C_{10}^{0}+C_{10}^{1}+C_{10}^{2}+\ldots+C_{10}^{10}-$ $-C_{10}^{0}-C_{10}^{1}-C_{10}^{2}=2^{10}-1-10-45=968$.

Answer: 968 sound combinations.