Giải thích các bước giải:
$\dfrac{\left ( n - 1 \right ).n.\left ( n + 1 \right )}{4} + \dfrac{n.\left ( n + 1 \right )}{2}$
$= \dfrac{\left ( n - 1 \right ).n.\left ( n + 1 \right )}{4} + \dfrac{2n.\left ( n + 1 \right )}{4}$
$= \dfrac{\left ( n - 1 \right ).n.\left ( n + 1 \right ) + 2n.\left ( n + 1 \right )}{4}$
$= \dfrac{\left ( n^{2} - n \right ).\left ( n + 1 \right ) + 2n.\left ( n + 1 \right )}{4}$
$= \dfrac{\left ( n^{2} - n + 2n \right ).\left ( n + 1 \right )}{4}$
$= \dfrac{\left ( n^{2} + n \right ).\left ( n + 1 \right )}{4}$
$= \dfrac{n.\left ( n + 1 \right ).\left ( n + 1 \right )}{4}$
$= \dfrac{n.\left ( n + 1 \right )^{2}}{4}$
