A) 10th
B) 9th
C) 8th
D) All of these
E) None of these
Correct Answer: B
Solution :
Explanation Option [b] is correct. Clearly, \[5\times 2=10,\text{ }10\times 2=20,\text{ }20\times 2-40,\text{ }......\] So, the series is a G.P. in which a = 5 and r = 2. Let 1280 be the nth term of the series. Then, \[5\times {{2}^{n-1}}=1280\Leftrightarrow {{2}^{n-1}}=\,\,256={{2}^{8}}\Leftrightarrow n-1\text{ }=\text{ }8\Leftrightarrow n=9.\]You need to login to perform this action.
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