This problem is for my algebra for elementary teachers class and I am stumped!
I am looking for an explicit formula
One line of triangles takes 3 toothpicks
Two lines of triangles takes 9 toothpicks
Three three lines of triangles takes 18 toothpicks
How many toothpicks does it take to make 4? Can you find a pattern?
Hint: The differences between the terms form an arithmetic series