Good luck with that. Reason it out!
Given two sides of a triangle, there are relatively obvious limitations on the size of the third side.
Try 2, and 3
Now, let's ponder the 3rd side.
Will 0 make a triangle? No. Greater than that.
Will 1 make a triangle? You tell me.
Will 2 make a triangle? You tell me.
Will 3 make a triangle? You tell me.
Will 4 make a triangle? You tell me.
Will 5 make a triangle? You tell me.
Will 6 make a triangle? No. Less than that.
Hmmm... What's the rule for that?