题目应该是这样的：
原式的=
2(3+1)(3^2 +1)(3^4 +1)(3^8 +1).(3^2n+1)/2
=(3-1)(3+1)(3^2 +1)(3^4 +1).(3^2n+1)/2
=(3^2-1)(3^2+1)(3^4+1).(3^2n+1)/2
=(3^4-1)(3^4+1)(3^8+1).(3^2n+1)/2
=.
=(3^4n-1)/2