\(N=1.2.3+2.3.4+...+n\left(n+1\right)\left(n+2\right)\)
\(4N=1.2.3.4+2.3.4.4+...+4n\left(n+1\right)\left(n+2\right)\)
\(4N=1.2.3.4+2.3.4.\left(5-1\right)+....+n\left(n+1\right)\left(n+2\right)\left[\left(n+3\right)-\left(n-1\right)\right]\)
\(4N=1.2.3.4+2.3.4.5-1.2.3.4+...+n\left(n+1\right)\left(n+2\right)\left(n+3\right)-\left(n-1\right)n\left(n+1\right)\left(n+2\right)\)
\(4N=n\left(n+1\right)\left(n+2\right)\left(n+3\right)\)
\(4N+1=n\left(n+1\right)\left(n+2\right)\left(n+3\right)+1\)
\(=n\left(n+3\right)\left(n+1\right)\left(n+2\right)+1\)
\(=\left(n^2+3n\right)\left(n^2+2n+n+2\right)+1\)
\(=\left(n^2+3n\right)\left(n^2+3n+2\right)+1\)
\(=\left(n^2+3n+1-1\right)\left(n^2+3n+1+1\right)+1\)
\(=\left(n^2+3n+1\right)^2-1+1=\left(n^2+3n+1\right)^2=t^2\)(1 số bất kì thỏa mãn)
Vậy \(4N+1\) là số chính phương (đpcm)
