Question

дайте ответ на эту задачу

Answer 1

$\displaystyle\bf\\\Big(A^{7} _{l} +A^{6} _{l} \Big):A^{5} _{l} =\Big(\frac{l!}{(l-7)!} +\frac{l!}{(l-6)!}\Big):\frac{l!}{(l-5)!}=\\\\\\=\frac{l!}{(l-7)!}\cdot\frac{(l-5)!}{l!}+ \frac{l!}{(l-6)!}\cdot\frac{(l-5)!}{l!} =\\\\\\=\frac{(l-5)!}{(l-5)!\cdot(l-6)\cdot(l-7)} +\frac{(l-5)!}{(l-5)!\cdot(l-6)}=\\\\\\=\frac{1}{(l-6)\cdot(l-7)} +\frac{1}{l-6} =\frac{1+l-7}{(l-6)\cdot(l-7)} =\\\\\\=\frac{l-6}{(l-7)\cdot(l-6)} =\frac{1}{l-7}$