Students are very interested in computers, the Internet and mobile phones. Using new technologies in the classroom can be very helpful for them.

    Mobile phones can be very useful in the classroom. When students speak or act, teachers can use them to record students, take photos and show them to students. Using mobile phones in class is a fun way of doing some interesting language work.

    How to use the Internet and mobile phones in class? Please remember the following key points (要点) :

    Use the Internet with your students in many different ways, if possible.

    Try to find good websites for your students.

    Use your mobile phone in class to record your students, take photos and play recordings.

    Ask your students to listen for the pronunciation mistakes when you play the recordings back to them.

先化简，再求值：

4xy﹣（x²+5xy﹣y²）﹣（x²+3xy﹣2y²），其中x=- $\frac{1}{4}$ ，y=- $\frac{1}{2}$ ．