1.
（1）A+B
=(1-5x²)+(4xy-x²-2)
=1-5x²+4xy-x²-2
=-6x²+4xy-1
（2）2A-3B
=2(1-5x²)-3(4xy-x²-2)
=2-10x²-12xy+3x²+6
=-7x²-12xy+8
2.
这个多项式是(mn+2n-4m²)-(3mm-3m²)
=mn+2n-4m²-3mm+3m²
=-2mn+2n-m²