Look at the following matchstick pattern of triangles.

Observe the given matchstick pattern and find the rule that gives the total number of matchsticks in terms of the number of triangles.

By observing the given pattern,

The number of matchsticks required to make 1, 2, 3, 4, ... triangles are 3, 5, 7, 9, ... respectively.

And the number of matchsticks required can be rewritten as, 2 + 1, 4 + 1, 6 + 1, 8 + 1 and so on.....

which is "one more than double the number of triangles" in the pattern.

So, the rule for the given pattern is 2n + 1, where n is the number of triangles.