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A group of students is arranging squares into layers to create a project. The first layer has 6 squares. The second layer has 12 squares. Which formula represents an arithmetic explicit formula to determine the number of squares in each layer?

a1 = 6; an = 6 + a(n − 1), n > 0
a1 = 6; an = 6 ⋅ a(n − 1), n > 0
a1 = 6; an = 6 ⋅ a(n + 1), n > 0
a1 = 6; an = 6 + a(n + 1), n > 0