For the problem of finding laws, we must first find out which parts have changed and according to what laws.

Solution: Let the matchstick used to extinguish the nth pattern be S N.

① Figure, s1= 4;

② Figure, S2 = 4+3× 4-(1+3) = 4+2× 4 = 4 (1+2);

③ Figure, S3 = 4 (1+2)+5× 4-(3+5) = 4 (1+2+3);

…;

The nth mode, sn = 4 (1+2+3+…+n-1)+(2n-1) × 4-(2n-3+2n-1) = 4 (/kloc-0).

= 4( 1+2+3+…+n- 1)+4n = 4( 1+2+3+…+n- 1+n)= 4×= 2n(n+ 1)。