(2a-b+3)(2a+b-3)-(a-2b-3)的平方
=[2a-(b-3)][2a+(b-3)]-[(a-2b)-3]^2
=4a^2-(b-3)^2-(a-2b)^2+6(a-2b)-9
=4a^2-b^2+6b-9-a^2+4ab-4b^2+6a-12b-9
=3a^2+6a+4ab-6b-5b^2-18
(2a-b+3)(2a+b-3)-(a-2b-3)的平方
=[2a-(b-3)][2a+(b-3)]-[(a-2b)-3]^2
=4a^2-(b-3)^2-(a-2b)^2+6(a-2b)-9
=4a^2-b^2+6b-9-a^2+4ab-4b^2+6a-12b-9
=3a^2+6a+4ab-6b-5b^2-18