f^(-1)(x)=log3(x)f^(-1)(8)=log3(8)=a+2a=log3(8)-2=log3(8/9)y=3^(ax)-4^x=3^[log3(8/9)x]-4^xy为减函数y(0)=0y(1)=8/9-4=-28/9值域[-28/9,0]