(3+1)(3^2+1)(3^4+1)（3^8+1)...(3^2n+1)
=(3-1)(3+1)(3^2+1)(3^4+1)（3^8+1)...(3^2n+1)/(3-1)
=(3^2-1)(3^2+1)(3^4+1)（3^8+1)...(3^2n+1)/2
=(3^4-1)(3^4+1)（3^8+1)...(3^2n+1)/2
……
=(3^4n-1)/2