a level maths sequences Watch

farrahparkinson4
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Just wondering on 4b how they got the a and r values, as i assumed for example that a would = 1, from part a, which it isn't. Thanks.Name:  question4b.jpg
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dextrous63
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Un isn't the GP, so u1 isn't a.

The GP is created by the numerator.
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Teenie2
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The sequence in part (a) isn't a true geometric sequence so you can't extract a value for a and r from it. What they've done is expand the sum of the first 15 terms and separated them into a geometric part and an arithmetic part. The geometric part has a = 2 and r = (2/3) , and the arithmetic part has 15 times -1 which totals -15. This then allows you to find the total.
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