3a²-[7a²-2a-3（a²-a）+1]
=3a²-【7a²-2a-3a²+3a+1】
=3a²-4a²-a-1
=-a²-a-1
（8-x²y+7xy²-6xy)-[8xy-(x²y+y²x)]
=8-x²y+7xy²-6xy-8xy+x²y+xy²
=8+8xy²-14xy