a^2b-[2(a^2b-2a^c)-(2b^2c-a^2c)]=a^2b-2(a^2b-2a^c)+(2b^2c-a^2c)=a^2b-2a^2b+4a^2c+2b^2c-a^2c=-a^2b+3a^2c+2b^2c请参考
