\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+...+\dfrac{1}{a.\left(a+1\right).\left(a+2\right)}=\dfrac{a.\left(a+3\right)}{4.\left(a+1\right)\left(a+2\right)}\)

Can you for me!

Calculate :

\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.3}+.....+\dfrac{1}{a+\left(a+1\right)+\left(a+2\right)}\)