Find the general term of this sequence ,

0 , 5 , 6 , 11 , ...

i can see the pattern +5 , +1 , +5 , ...

So i hv this idea ..

$\displaystyle 3+(-1)^r(2) $

where

$\displaystyle U_1=0 $

$\displaystyle U_2=U_1+3+(-1)^r(2)$ , where r=2

$\displaystyle U_3=U_2+3+(-1)^r(2) $, where r=3

And so on ..

$\displaystyle

U_n=U_{n-1}+\sum^{n}_{r=1}[3+(-1)^r(2) ]

$

I am not sure how to continue .. Am i on the right track . Thanks !