1. The problem statement, all variables and given/known data an arithmetic progression(a1-a9) has 9 numbers. a1 equals 1 The combination(S) of all of the numbers of the arithmetic progression is 369 a geometric progression(b1-b9) also has 9 numbers. b1 equals a1(1) b9 equals a9(unknown) find b7 2. Relevant equations 3. The attempt at a solution basically I use Sn = ((2*a1 + (n-1)*d)/2)*n and I get 369 = 9 + 36*d; d = 10 then I find a9: a9 = a1 + 8*d a9 = 1 + 80 = 81; and I know b9 equals a9, so b9 = 81 then with the formula for the geometric progression I do: bn = b1*q^(n-1) b9 = 1*q^8 81 = q^8 9 = q^7 3 = q^6; which should be b7, however in the book's answers, it's not '3', but '27'.How is that even possible if b1 is said to be '1'?