What is the rule that gives the number of matchsticks in terms of the number of triangles, as shown in the figures below?
The column of 'number of matchsticks - 1' forms the multiplication table of 2, i.e., 2, 4, 6, 8, ...
Let the number of triangles be x.
The number of matchsticks used in each pattern is 1 more than 2 times the number of triangles in it.
∴The number of matchsticks used in each pattern = 2x+1