323 =17.19.
Ta có: \(20^n+16^n-3^n-1=\left(20^n-3^n\right)+\left(16^n-1\right)\)
\(20^n-3^n⋮17,16^n-1⋮17\)(vì n chẵn)
\(\Rightarrow20^n+16^n-3^n-1⋮17\)(1)
Tương tự:
\(20^n+16^n-3^n-1=\left(20^n-1\right)+\left(16^n-3^n\right)\)
\(20^n-1⋮19,16^n-3^n⋮19\)(vì n chẵn)
\(\Rightarrow20^n+16^n-3^n-1⋮19\)(2)
Từ (1) và (2) \(\Rightarrow20^n+16^n-3^n-1⋮\left(17,19\right)=323\)(đpcm)