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