- #1
Hivoyer
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Homework Statement
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
Homework Equations
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'?