计算，能用简便算法的就用简便算法．

$\frac{9}{10}-\frac{7}{10}\times \frac{5}{21}=$ ____

秦国成为战国七雄中实力最强的国家，并统一了六国，主要原因是（    ）

任务型阅读
     I go to a birthday party on October 10th.It is for my friend，Alice. She is an English girl. But she learns Chinese in China. She is in my class. I buy a sweater for her. Lots of people come to the party. We play games. The happy thing for me is to eat. I eat a lot. When I get back home(回到家)，I am not well. I think I eat too much(太多)．Next time(下次)I will eat only one hamburger，not five.

lose(过去式) ____

如图，在Rt△ABC中，∠ACB=90°．