19.3（-2ab+3a）-（2a-b)+6ab= 1/2a^2-[(ab-a^2)+4ab]-1/2ab=
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3（-2ab+3a）-（2a-b)+6ab
=-6ab+9a-2a+b+6ab
=7a+b
1/2a^2-[(ab-a^2)+4ab]-1/2ab
=1/2a^2-(ab-a^2)-4ab-1/2ab
=1/2a^2-ab+a^2-4ab-1/2ab
=3/2a^2-11/2ab
(3a^2b+1/4ab^2)-(3/4ab^2+a^2b)
=3a^2b+1/4ab^2-3/4ab^2-a^2b
=2a^2b-1/2ab^2
=-6ab+9a-2a+b+6ab
=7a+b
1/2a^2-[(ab-a^2)+4ab]-1/2ab
=1/2a^2-(ab-a^2)-4ab-1/2ab
=1/2a^2-ab+a^2-4ab-1/2ab
=3/2a^2-11/2ab
(3a^2b+1/4ab^2)-(3/4ab^2+a^2b)
=3a^2b+1/4ab^2-3/4ab^2-a^2b
=2a^2b-1/2ab^2
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