The nth term of a GP 18, -12, 8,...... is 512729. The value of n is .
A
7
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B
8
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C
9
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D
10
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Solution
The correct option is C 9 The given GP 18, -12, 8,... The first term of the GP is 18 and the common ratio is r=−1218=−23an=512729⇒arn−1=512729⇒18(−23)n−1=512729⇒(−23)n−1=512729×18=2936×2×32⇒(−23)n−1=2838=(−23)8⇒n−1=8⇒n=9