Small clarification: applying Coulomb's Law

[UselessLadder]

Homework Statement

Hi,

I'm studying electric charges and fields, and I have a quick question about this example in my book:

Everything is straightforward except the subscripts (13 and 23) on r in the denominators of initial Coulomb equations for F₁ and F₂. In the next step they just plugged in the distances, so those numbers don't seem to do anything, and there are no subscripts in the original formula. So what are they?

Homework Equations

F = k [(q₁q₂)/r²]

 

[bowlbase]

They just indicate that the force one is from charge 1 to charge 3. And the force 2 is from charge 2 to charge 3.

 

[UselessLadder]

Duh, I knew it had to be something obvious. Thank you!