(3+1)(3^2+1)(3^4+1)(3^8+1)...(3^2n+1)
=(3^2-1)(3^2+1)(3^4+1)(3^8+1)...(3^2n+1)/(3-1)
=(3^4-1)(3^4+1)(3^8+1)...(3^2n+1)/(3-1)
=(3^8-1)(3^8+1)...(3^2n+1)/(3-1)
=(3^16-1)...(3^2)^n+1)/(3-1)
=[(3^2)^2n+1]/2
=(81^n +1)/2
(3+1)(3^2+1)(3^4+1)(3^8+1)...(3^2n+1)
=(3^2-1)(3^2+1)(3^4+1)(3^8+1)...(3^2n+1)/(3-1)
=(3^4-1)(3^4+1)(3^8+1)...(3^2n+1)/(3-1)
=(3^8-1)(3^8+1)...(3^2n+1)/(3-1)
=(3^16-1)...(3^2)^n+1)/(3-1)
=[(3^2)^2n+1]/2
=(81^n +1)/2