Ta có: \(\left(3+1\right)\left(3^2+1\right)\left(3^4+1\right)\cdot...\cdot\left(3^{64}+1\right)\)
\(=\dfrac{\left(3^2-1\right)\left(3^2+1\right)\left(3^4+1\right)\cdot...\cdot\left(3^{64}+1\right)}{2}\)
\(=\dfrac{\left(3^4-1\right)\left(3^4+1\right)\cdot...\cdot\left(3^{64}+1\right)}{2}\)
\(=\dfrac{\left(3^8-1\right)\left(3^8+1\right)\left(3^{16}+1\right)\left(3^{32}+1\right)\left(3^{64}+1\right)}{2}\)
\(=\dfrac{\left(3^{16}-1\right)\left(3^{16}+1\right)\left(3^{32}+1\right)\left(3^{64}+1\right)}{2}\)
\(=\dfrac{\left(3^{32}-1\right)\left(3^{32}+1\right)\left(3^{64}+1\right)}{2}\)
\(=\dfrac{\left(3^{64}-1\right)\left(3^{64}+1\right)}{2}\)
\(=\dfrac{3^{128}-1}{2}\)
\(A=4\left(3^2+1\right)\left(3^4+1\right)\cdot\cdot\cdot\left(3^{64}+1\right)\)
\(\Rightarrow2A=\left(3^2-1\right)\left(3^2+1\right)\left(3^4+1\right)\cdot\cdot\cdot\left(3^{64}+1\right)\)
\(=\left(3^4-1\right)\left(3^4+1\right)\cdot\cdot\cdot\left(3^{64}+1\right)\)
\(=3^{128}-1\)
\(\Rightarrow A=\dfrac{3^{128}-1}{2}\)
