A=2B+CC＝A－坦答2B＝（御信虚镇燃a^3-2a^2b)-2(3a^2b+2ab^2-a^2)=a^3-2a^2b-6a^2b-4ab^2+2a^2=a^3-8a^2b-4ab^2+2a^2
C=2B-A=6a²b+4ab²-a²-a³+2a²b=8a²b+4ab²-a²-a³
