Question

A pattern is made by toothpick squares. Select the series that represents the number of toothpicks added to a picture to get the next picture to the right. Pretend the first picture has no toothpicks.



4 + 8 + 12 + 16 + ...; rule is a_n = 4n
1 + 2 + 3 + 4 + ...; rule is an = n
1 + 4 + 9 + 16 + ...; rule is an = n2
4 + 12 + 24 + 40 + ...; rule is an = 2(n2 + n)