РЕШИТЕ УРАВНЕНИЕ (Х+1)!/20=(х-1)!

[latex]\frac{(x+1)!}{20} = (x-1)!; x \in \mathbb{N} \\ n! = n(n-1)! \\ \frac{x(x+1)(x-1)!}{20} = (x-1)! |:(x-1)! \\ \frac{x(x+1)}{20} = 1 | 20 \\ x(x+1) = 20 \\ x(x+1) = 4*5 \\ x = 4 [/latex]