20.已知A=x2+xy+y2,B=-3xy-x2.计算:
(1)A-B;
(2)2A-3B.
分析 (1)、(2)直接把A=x2+xy+y2,B=-3xy-x2代入进行计算即可.
解答 解:(1)∵A=x2+xy+y2,B=-3xy-x2,
∴A-B=(x2+xy+y2)-(-3xy-x2)
=x2+xy+y2+3xy+x2
=2x2+4xy+y2;
(2)∵A=x2+xy+y2,B=-3xy-x2,
∴2A-3B=2(x2+xy+y2)-3(-3xy-x2)
=2x2+2xy+2y2+9xy+3x2
=5x2+11xy+2y2.
点评 本题考查的是整式的加减,熟知整式的加减实质上就是合并同类项是解答此题的关键.