-2a^2-[-4a^2-(-a)^2],其中a=-1
=-2a^2+4a^2+a^2
=3a^2
=3×（-1）^2
=3
2（a+b）-（a-b）-2/3(a+b)-1/2a+1/2b,其中a=-5,b=-1
=2（a+b）-2/3(a+b)-[（a-b）+1/2(a-b)]
=4/3(a+b)-3/2(a-b)
=4/3 (-5-1)-3/2 (-5+1)
=-8+6
=-2