Xét :\(D=\left(3+1\right)\left(3^2+1\right)\left(3^4+1\right)\left(3^8+1\right)\)
\(=\left(3-1\right)\left(3+1\right)\left(3^2+1\right)\left(3^4+1\right)\left(3^8+1\right).\frac{1}{2}\)
\(=\left(3^2-1\right)\left(3^2+1\right)\left(3^4+1\right)\left(3^8+1\right).\frac{1}{2}\)
\(=\left(3^4-1\right)\left(3^4+1\right)\left(3^8+2\right).\frac{1}{2}\)
\(=\left(3^8-1\right)\left(3^8+1\right).\frac{1}{2}\)
\(=\left(3^{16}-1\right).\frac{1}{2}\)
Vì \(\frac{3^{16}-1}{2}< 3^{16}-1\)
nên D < C
Vậy D < C