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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?

a. a1 = 6; an = 6 + an − 1, n > 0
b. a1 = 6; an = 6 ⋅ an − 1, n > 0
c. a1 = 6; an = 6 ⋅ an + 1, n > 0
d. a1 = 6; an = 6 + an + 1, n > 0