题目

Sometimes people come into your life and you know at once that they were sure to be there.They serve some sort of purpose(目的)，teach you a lesson or help find out who you are or who you want to become.You never know who these people may be：your classmate，neighbor，teacher，long­lost friend or even a real stranger.But you know that every moment they will affect(影响) your life in some serious way. Sometimes things happen to you.At the time they may seem terrible，painful(痛苦的) and unfair.But later you realize that without getting over those difficulties you would ha（2004•虹口区一模）在△ABC中，记外接圆半径为R．（1）求证：2Rsin(A-B)=a2-b2c；（2）若（a2+b2）sin（A-B）=（a2-b2）sin（A+B），试判断△ABC的形状．