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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