a=n(n+1)(n+2)(n+3)+1
=n(n+3)(n+1)(n+2)+1
=(n²+3n)(n²+3n+2)+1
=(n²+3n)²+2(n²+3n)+1
=(n²+3n+1)
4×5×6×7+1=841=29²=(4²+3×4+1)²
n(n+k)(n+2k)(n+3k)+k⁴
=n(n+3k)(n+k)(n+2k)+k⁴
=(n²+3kn)(n²+3kn+2k²)+k⁴
=(n²+3kn)²+2k²(n²+3kn)+(k²)²
=(n²+3kn+k²)²
3×5×7×9+2⁴=945+16=961=31²=(3²+3×2×3+2²)²