Question

Найдите: 1)A^4 7; 2)A^4 5​

Answer 1

$\displaystyle A^k_n=\frac{n!}{(n-k)!}$

$1)\;\;\displaystyle A^4_7=\frac{7!}{(7-4)!} =\frac{7*6*5*4*3!}{3!} =7*6*5*4=840 \\\\\\2)\;\;\displaystyle A^4_5=\frac{5!}{(5-4)!}=\frac{5!}{1!} =\frac{5!}{1} =5!=5*4*3*2*1=120$