解；原式=(9a²b²*a³-2a²*27a³b³*1/2b)/(9a²b²)=(1*a³-2a²*3ab*1/2b)=a³-3a³b²=a³(1-3b²)=1/27x(1-3x1/4)=1/27x1/4=1/108