Summation question

[NameIsUnique]

I know n(n+1)/2 solves from 1.... n by 1's

Is there a formula where you can add up by 3's?

Example: 3 + 6 + 9 + 12 ... n

 

[Samy_A]

I know n(n+1)/2 solves from 1.... n by 1's

Is there a formula where you can add up by 3's?

Example: 3 + 6 + 9 + 12 ... n

Hint: 3+6+9+12=3(1+2+3+4)