Let f(n)=∣∣
∣
∣∣nn+1n+2nPnn+1Pn+1n+1Pn+2nCnn+1Cn+1n+1Cn+2∣∣
∣
∣∣
Then, f(n) is divisible by
n2+n+1
n!
f(n)=∣∣ ∣ ∣∣nn+1n+2nPnn+1Pn+1n+2Pn+2nCnn+1Cn+1n+2Cn+2∣∣ ∣ ∣∣=n!∣∣ ∣ ∣∣n(n+1)(n+2)1(n+1)(n+1)(n+2)111∣∣ ∣ ∣∣Δ=n!∣∣ ∣ ∣∣n111n(n+1)2100∣∣ ∣ ∣∣(C3→C3−C2,C2→C2−C1)Δ=n!×[(n+1)2−n]=n!×(n2+n+1)