\(\left(x+1\right).\left(x+2\right).\left(x+3\right).\left(x+4\right)-24\)
\(=[\left(x+1\right).\left(x+4\right)].[\left(x+2\right).\left(x+3\right)]-24\)
\(=\left(x^2+4x+x+4\right).\left(x^2+3x+2x+6\right)-24\)
\(=\left(x^2+5x+4\right).\left(x^2+5x+6\right)-24\)
Ta đặt \(n=x^2+5x+4\)
Lúc này biểu thức trở thành \(n.\left(n+2\right)-24\)
\(=n^2+2n-24\)
\(=n^2+2n+1-25\)
\(=\left(n+1\right)^2-5^2\)
\(=\left(n+1-5\right).\left(n+1+5\right)\)
\(=\left(n-4\right).\left(n+6\right)\)
\(=\left(x^2+5x+4-4\right).\left(x^2+5x+4+6\right)\)
\(=\left(x^2+5x\right).\left(x^2+5x+10\right)\)
