I get stuck for both AP n GP when the first term isnt given. EG
1. The second term of an AP is 15 and the fifth term is double the first. Find the sum of the first 20 terms of the series.
(a)780 (b)810 (c)720 (d)790 (e)805
2. Find the series in the GP whose 5th term is 80 and the 8th term is 640.
(a)5, 10, 20, 50 (b)2, 4, 6, 8 (c)5, 7, 9, 11 (d)5, 10, 20, 40 (e)5, 20, 40, 80
I figured the answer to the second one is D. I need to know whether this is based on trial and error alone or is there a specific formula to be used here? How do u know what's the difference between the terms or the ratio if only the second term and/or some other random term is given??
2007-07-29
03:35:15
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4 answers
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asked by
sampan_50
1
in
Mathematics