（a-2b+c）（a+2b-c）—（a+2b+c）^2
=[a-(2b-c)][a+(2b-c)]-(a+2b+c)^2
=a^2-(2b-c)^2-(a+2b+c)^2
=[a^2-(a+2b+c)^2]-(2b-c)^2
=(a+a+2b+c)(a-a-2b-c)-(2b-c)^2
=(2a+2b+c)(-2b-c)-(2b-c)^2
=-4ab-2ac-4b^2-2bc-2bc-c^2-4b^2+4bc-c^2
=-4ab-2ac-8b^2-2c^2