第一个：（2a-b)^2*(2a-b)
=(4a^2+b^2-4ab)*(2a-b)
=8a^3+2ab^2-8a^2b-4a^2b-b^3+4ab^2
=8a^3+6ab^2-12a^2b-bb^3
第二个：=3a*(3a-b+c)-b*(3a-b+c)+c*(3a-b+c)
=9a^2-3ab+3ac-3ab+b^2-bc+3ac-bc+c^2
=9a^2-6ab+6ac-2bc+b^2+c^2
第三个：=-[a*(a+3b-2c)+3b*(a+3b-2c)+2b*(a+3b-2c)]
=-[a^2+3ab-2ac+3ab+9b^2-6bc+2ac+6bc-4c^2]
=4c^2-a^2-6ab-9b^2