Matchsticks need 1*3 for the first time, 3*3 for the second time, 6*3 for the third time and 10*3 for the fourth time. ...

The first requirement is 1*3 matching, the second requirement is (1+2)*3 matching, the third requirement is (1+2+3)*3 matching, and the fourth requirement is (1+2+3+4) *. ...

( 1+2+3+4+……+n)*3

=(n*(n- 1))*3/2

The nth graph needs (n*(n- 1))*3/2.

(A match) has n 2 triangles.