下面这个方法较简单：a(n+1)=3an+3^(n+1),两边同除以3^(n+1)可得：a(n+1)/ 3^(n+1)= 3an/ 3^(n+1)+1,a(n+1)/ 3^(n+1)= an/ 3^n+1,设an/ 3^n=bn,则b(n+1)=bn+1,这说明数列{bn}是公差为1的等差数列,首项为b1=a1/3=1.bn...