已知$\alpha$为第二象限角，则$\frac{2\sin \alpha }{\sqrt{1-{\cos }^2\alpha }}+\frac{\sqrt{1-{\sin }^2\alpha }}{\cos \alpha }$的值是（  ）

$24\times (\frac{3}{8}+\frac{5}{6})=$ ____

设${\left(5x-\frac{1}{\sqrt{x}}\right)}^n$的展开式的各项系数和为M，二项式系数和为N，若$M-N=240$，则展开式中x的系数为  （   ）

    Mr. Green and his family decided to live in Beijing for two weeks.

    One morning they left for the Great Wall by bus at 9:00. It took them about one hour and a half to get there. Then they visited the Great Wall and took lots of photos. Four hours later it began to rain. They had to go back to the hotel. After getting to the hotel, they found their camera was lost. Mr Green's kids felt sorry and they didn't want to eat anything for dinner. Mr Green told a joke to make them happy. “An old man lived in a tall building. It had sixty storeys. One day he fell down from it, but he didn't hurt himself,” Mr Green said. “Do you know why?”

    The kids were surprised and they didn't know the answer.

     “Because he lived on the first floor.”

     The kids all laughed after hearing the answer. Just then a policeman came in with a bag. She said a Chinese boy found the bag on the Great Wall and then found them. Mr Green opened the bag and found the camera. They all became very happy.