（1）x的二次方加2xy加3减2xy减6
=x²+2xy+3-2xy-6
=x²-3
(2)ax加3a的二次方加x的二次方-3ax-4-5a的二次方-2x的二次方-6
=ax+3a²+x²-3ax-4-5a²-2x²-6
=-2a²-x²-2ax-10
（3）-1/2x的平方y-2/3X的三次方+5-4/3X的三次方-7+2x的平方y
=-1/2x²y-2/3x³-4/3x³+2x²y+5-7
=-3x³+3/2x²y-2
（4）0.2x的平方-0.3x+0.5-0.1x的平方+0.7x-0.11
=0.2x²-0.3x+0.5-0.1x²+0.7x-0.11
=0.1x²+0.4x+0.39