a1 = first term a1 = 15 --------------------------------------------- d = common difference d = (second term) - (first term) d = a2 - a1 d = 9-12 d = -3 The negative common difference indicates that the terms are decreasing Each time we subtract 3, or add -3, to get the next term. ---------------------------------------------- nth term of arithmetic sequence an = a1 + d(n-1) an = 15 + (-3)(n-1) an = 15- 3n + 3 an = 18 - 3n
