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a = - 8 Ratio of the 7th and 9th term is a + 6d : a + 8d = 5:8 8(a+6d) = 5(a+8d) 8a+48d=5a+40d 48d - 40d = 5a + 8a 8d = -3a d = -3a÷8 d = -3×(-8)÷8 = 3 Therefore common difference = 3
8 - 5 is 3 ie 3 is the common difference
