3a³-a=1则有3a（a²-1）=1
9a^4+12a³-3a²-7a+2012
=9a^4-3a+12a³-4a-3a²+2012
=（9a^4-3a）+（12a³-4a）-3a²+2012
=3a（3a²-1）+4（3a³-a）-3a²+2012
=3a*a+4-3a²+2012
=2016