设y=log3 (3^x-1)
则y-1=log3(3^(x-1)-1/3)
则y*(y-1)=2
y=2或=-1
当y=2,
3^x-1=9
x=log3 10
当y=-1
3^x-1=1/3
3^x=4/3
x=(log3 4)-1