ab滿足|a-2|+(b+1)平方=0
a-2=0 b+1=0
a=2 b=-1
5ab平方-{2a平方b-[3ab平方-（4ab平方-2ab平方)]},
=5ab^2-{2a^2b-[3ab^2-4ab^2+2ab^2]}
=5ab^2-{2a^2b-ab^2}
=5ab^2-2a^2b+ab^2
=6ab^2-2a^2b
=6*2*(-1)^2-2*2^2*(-1)
=12+8
=20