`8^10-8^9-8^8`

`=8^8(8^2-8-1)`

`=8^8(64-8-1)`

`=55.8^8 vdots 55`

`=>` Chọn 55

A

là A vì đáp án ấy thỏa mãn điều kiện "lớn hơn 6 và nhỏ hơn 12".

A

\(\dfrac{4}{3}\cdot\dfrac{9}{8}\cdot\dfrac{16}{15}\cdot...\cdot\dfrac{400}{399}\)
\(=\dfrac{2\cdot2}{1\cdot3}\cdot\dfrac{3\cdot3}{2\cdot4}\cdot\dfrac{4\cdot4}{3\cdot5}\cdot...\cdot\dfrac{20\cdot20}{19\cdot21}\)
\(=\dfrac{2\cdot2\cdot3\cdot3\cdot4\cdot4\cdot...\cdot20\cdot20}{1\cdot3\cdot2\cdot4\cdot3\cdot5\cdot...\cdot19\cdot21}\)
\(=\dfrac{2\cdot3\cdot4\cdot...\cdot20}{1\cdot2\cdot3\cdot...\cdot19}\cdot\dfrac{2\cdot3\cdot4\cdot...\cdot20}{3\cdot4\cdot5\cdot...\cdot21}\)
\(=20\cdot\dfrac{2}{21}\)
\(=\dfrac{40}{21}\)

 Tham khảo nha bn:

        I think the best recreational activity for teenagers is playing sports. First, they can find friends with similar interests. Second, physical activity is a good way to reduce stress and reduce depression. Furthermore, sport can help develop teamwork and leadership skills that may be necessary for their future jobs. Their parents should offer advice on choosing an appropriate recreational activity; However, it is the youth who make the final decision.