Right =[n*(n- 1)！ ]/[(n- 1)-(m- 1)]！
=n！ /(n-m)！ ,
∴ A(m，n)=nA(m- 1，n- 1)
2)、A(8，8)-8*A(7，7)+7*A(6，6)
=8! -8*7! +7*6!
=8! -8! +7!
=7!
=A(7，7)
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