Задача 7.

For $n>0$ compute the following 1) $1\cdot2 + 2\cdot3 + \ldots + (n-1)\cdot{n}$; 2) $\frac{1}{4\cdot5} + \frac{1}{5\cdot6} + \ldots + \frac{1}{(n+3)(n+4)}$; 3*) $1\cdot2\cdot3 + 2\cdot3\cdot4 + \ldots + n\cdot(n+1)\cdot(n+2)$.