已知a=2x²-3xy+y²,b=x²+2xy-3y²,化简：(1)a+b (2)a-2b.请把过程也写出来,

已知a=2x²-3xy+y²,b=x²+2xy-3y²,化简：(1)a+b (2)a-2b.请把过程也写出来,

a+b
=2x²-3xy+y²+x²+2xy-3y²
=3x²-xy-2y²
a-2b
=2x²-3xy+y²-2(x²+2xy-3y²)
=2x²-3xy+y²-2x²-4xy+6y²
=-7xy+7y²