13.已知M=3x的平方-2xy+y平方,N＝2x的平方＋xy－3y的平方求；（1）M-N:(2）M+N:

解(1)
M-N
=(3x²-2xy+y²)-(2x²+xy-3y²)
=3x²-2xy+y²-2x²-xy+3y²
=(3x²-2x²)+(-2xy-xy)+(y²+3y²)
=x²-3xy+4y²
(2)
M+N
=(3x²-2xy+y²)+(2x²+xy-3y²)
=3x²-2xy+y²+2x²+xy-3y²
=(3x²+2x²)+(-2xy+xy)+(y²-3y²)
=5x²-xy-2y²