Two triangles need 3+2=5 matches,

Three triangles need 3+2×2=7 matches,

Four triangles need 3+2×4= 1 1 matching.

…

10 triangles need 3+2× 10=23 matches.

Therefore, please fill in the following form:

Number of triangles 1 234 …/number of matchsticks used in kloc-0/0 3 5 7 9 … 23(2) N triangles from (1) need 3+2n matches.

So the answer is: (3+2n).