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