Đặt \(A=5\cdot7^{2\left(n+1\right)}+2^{3n}=5\cdot49^{n+1}+8^n=5\left(41+8\right)^{n+1}+8^n\)
Áp dụng công thức nhị thức Newton, ta có:
\(\left(41+8\right)^{n+1}=41^{n+1}+\left(n+1\right)\cdot41^n\cdot8+\dfrac{n\left(n+1\right)}{2}\cdot41^{n-1}\cdot8^2+...+\left(n+1\right)\cdot41\cdot8^n+8^{n+1}\)
Vậy \(A=5\left[41^{n+1}+\left(n+1\right)\cdot41^n\cdot8+..+\left(n+1\right)\cdot41\cdot8^n+8^{n+1}\right]+8^n\)
\(\Rightarrow A=5\left[41^{n+1}\left(n+1\right)\cdot41^n\cdot8+...+\left(n+1\right)\cdot41\cdot8^n\right]+5\cdot8^{n+1}+8^n\)
Đặt \(B=41^{n+1}\left(n+1\right)\cdot41^n\cdot8+...+\left(n+1\right)\cdot41\cdot8^n\)
\(\Rightarrow B⋮41\)
Đặt \(C=5\cdot8^{n+1}+8^n=8^n\left(5\cdot8+1\right)=8^n\cdot41\)
\(\Rightarrow C⋮41\)
Mà \(A=B+C\Rightarrow A⋮41\)
\(\RightarrowĐPCM\)
