已知(x-ay)(x+ay)=x2-9y2,那么 a=______.
∵(x-ay)(x+ay)=x2-a2y2,
∴a2=9,
解得a=±3.
故答案为:±3.
根据所提供的情景,选择相应的句子。
1. 你想知道对方是否会拉小提琴,可以说: [ ]
A. Shall we play the violin?
B. Do you play the violin?
C. What about the violin?
2. 你夸奖对方的吉他,可以说: [ ]
A. This is my guitar.
B. How nice the guitar is!
C. I'd like your guitar.
3. 你想问别人会玩什么,你可以说:[ ]
A. Can I help you?
B. What do you play?
C. What do you like?
4. 林涛问你是否会弹钢琴,你不会说:[ ]
A. Yes, I can.
B. No , I'm Lin Tao.
C. No, I can't.
5. 你想问别人是否喜欢足球,可以说:[ ]
A. Look at the football.
B. Do you play the football?
C. Do you like the football7
6. 请求别人一起踢足球,可以说:[ ]
A. What do we play?
B. Do you like football?
C. Shall we go and play football?
油环起什么作用?常用的气环有哪几种类型?