2012 AIME II Problems/Problem 2

Problem 2

Two geometric sequences $a_1, a_2, a_3, \ldots$ and $b_1, b_2, b_3, \ldots$ have the same common ratio, with $a_1 = 27$ , $b_1=99$ , and $a_{15}=b_{11}$ . Find $a_9$ .

Solution

Call the common $r.$ Now since the $n$ th term of a geometric sequence with first term $x$ and common ratio $y$ is $xy^{n-1},$ we see that $a_1 \cdot r^{14} = b_1 \cdot r^{10} \implies r^4 = \frac{99}{27} = \frac{11}{3}.$ But $a_9$ equals $a_1 \cdot r^8 = a_1 \cdot (r^4)^2=27\cdot \frac{11} 3^2=27\cdot \frac{121} 9=\boxed{363}$ . $\square$

2012 AIME II (Problems • Answer Key • Resources)
Preceded by Problem 1	Followed by Problem 3
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2012 AIME II Problems/Problem 2

Problem 2

Solution

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