### Solution

From the figure we observe that in the given matchstick pattern, the number of matchsticks are 3, 5, 7 and 9 which is 1 more than the twice of the number of triangles in the pattern.

The number of matchsticks used in each pattern is 1 more than 2 times the number of triangles in it.

Therefore the pattern is 2x + 1, where x is the number of triangles.