(a-2b+3c)(a+2b-3c)
=[a-(2b-3c)][a+(2b-3c)]
=a^2-(2b-3c)^2
=a^2-4b^2+12bc-9c^2
（2x+y）（2x-y）+（x+y）^2-2（2x^2-xy)
=（2x+y）（2x-y）+（x+y）^2-2x（2x-y)
=y（2x-y）+（x+y）^2
=2xy-y^2+x^2+2xy+y^2
=x^2+4xy