合并同类项

2x^2-3x+5-2x^2+3x-5
=(2-2)x^2+(3-3)x+(5-5)
=0
2xy-3yx-5x^2y-5yx^2
=-xy-5x^2-5yx^2
-[2a-b]-{b-[a-2a-b]}
=-2a+b-b+[-a-b]
=-2a-a-b
=-3a-b.