两边同时乘以3^x 得：
3^2 * (3^x)^2 - 3^2 =8*(3^x)
整理：(3^x)^2 - 8/9*(3^x) - 1 =0
==> (3^x)^2 - 8/9*(3^x) +(4/9)^2 = 1+(4/9)^2
==> ( 3^x - 4/9)^2 = 97/81
==> 3^x = 4/9 + √97/9 ；
或 3^x = 4/9 - √97/9 (3^x x =ln(4+√97)/ln3 -2
两边同时乘以3^x 得：
3^2 * (3^x)^2 - 3^2 =8*(3^x)
整理：(3^x)^2 - 8/9*(3^x) - 1 =0
==> (3^x)^2 - 8/9*(3^x) +(4/9)^2 = 1+(4/9)^2
==> ( 3^x - 4/9)^2 = 97/81
==> 3^x = 4/9 + √97/9 ；
或 3^x = 4/9 - √97/9 (3^x x =ln(4+√97)/ln3 -2