It's the end of class. When the bell rings, students of Luohu ForeignLanguages School in Shenzhen quickly take out their cell phones.
They want to log on to their micro blogs to check the interesting things that have happened in the last hour.
   Since last year, the trend (潮流) of micro blogging has swept the country. Recent surveys shows that most students in middle schools have a micro blog and some even update their blogs over five times per day.
   "We learn many fresh and interesting things on micro blogs and they have become popular topics in class," said Andy Liang, 14. "If you do not know about them, you are out of the loop."
   It is also a great place for students to let out stress. "My parents always ask me to study hard, and encourage me before exams, but it really adds pressure," said Simon Zhang, 15. "When I share these feelings on my micro blog, I get many replies from friends in the same situation, which makes me feel better."
   But parents are worried that micro blogging could be a waste of time. Some misleading messages may even cause danger to kids, they said.
   Mr. Shen, a professor at the China Education Association, suggests parents not worry too much as long as kids are not crazy about micro blogging. Maybe it can become a window for parents to understand their children.
   "If parents can read their children's micro blogs, they'll know their thoughts, thus leading to better communication and solutions to problems," he said. He also gives some tips for kids.
   -Don't micro blog for more than one hour a day.
   -Never micro blog in class.
   -Try to talk face to face with people from time to time.
   -Be critical(批判性的). Don't trust all the messages on a micro blog.

已知a、b是两条不重合的直线，α、β、γ是三个两两不重合的平面，给出下列四个命题：

①若a⊥α，a⊥β，则α∥β；

②若α⊥γ，β⊥γ，则α∥β；

③若α∥β，a⊂α，b⊂β，则a∥b；

④若α∥β，α∩γ=a，β∩γ=b，则a∥b．

其中正确命题的序号有____ 

已知直线y=2x与y=﹣x+b的交点为（﹣1，a），则方程组 $\left\{\begin{array}{c}y-2x=0\\ y+x-b=0\end{array}\right.$的解为（　　）