Find the least value of n for which the sum 1 + 3 + 32+ ...... t

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 Multiple Choice QuestionsShort Answer Type

81.

How many terms of the sequence

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82.

Sum the following series:

7 + 77 + 777 + ......... to n terms

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83.

Sum the following series:

0.5 + 0.55 + 0.555.......... to n terms



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84.

Given a G.P. with a = 729 and 7th term 64, determine S7.

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85.

Determine the number of terms in a GP., if t1 = 3, tn = 96 and Sn = 189.

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86.

Find the least value of n for which the sum 1 + 3 + 32+ ...... to n terms is greater than 7000.


Let space space straight S subscript straight n space equals space 1 plus 3 plus 3 squared plus...... space to space straight n space terms

Here,                a = 1, r = 3

                   straight S subscript straight n. space space space space fraction numerator straight a left parenthesis straight r to the power of straight n minus 1 right parenthesis over denominator straight r minus 1 end fraction space equals space fraction numerator 1 left parenthesis 3 to the power of straight n minus 1 right parenthesis over denominator 3 minus 1 end fraction space equals space fraction numerator 3 to the power of straight n minus 1 over denominator 2 end fraction

Since  straight S subscript straight n greater than 7000

∴       fraction numerator 3 to the power of straight n minus 1 over denominator 2 end fraction greater than 7000            rightwards double arrow space space space space space space space space space space space space space space space space space 3 to the power of straight n minus 1 space greater than space 14000       rightwards double arrow space space space space space space space space space space space 3 to the power of straight n greater than 14001

rightwards double arrow            3 to the power of straight n space greater than space 19683 space greater than space 14001 space greater than space 6561

rightwards double arrow             3 to the power of straight n greater than space 3 to the power of 9 greater than 14001 space greater than space 3 to the power of 8

Hence, the least value of n is 9.

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87.

The ratio of the sum of first three terms is to that of first 6 terms of a G.P. is 125 : 152. Find the common ratio.

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88.

If S1, S2 and S3 be respectively the sums of n, 2n and 3n terms of a GP., prove that S1 (S3 – S2) = (S2 – S1)2

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 Multiple Choice QuestionsLong Answer Type

89.

Let S be the sum, P the product and R the sum of the reciprocals of n terms in a G.P. Prove that straight P squared straight R to the power of straight n space equals space straight S to the power of straight n.

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 Multiple Choice QuestionsShort Answer Type

90.

Evaluate:

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