Q6 of 105 Page 146

If the sum of first 7 terms of an AP is 49 and that of 17 terms is 289, find the sum of first n terms.

This is not in GP

Since they have given common difference -2


But need here is common ratio in GP series


(OR)


They have said that successive steps need 2 bricks less than the previous.


If we write the number of bricks used for each steps


100, 100-2, 100-4,……100-48


100, 98, 96,…..52


Common ratio = r1 = = =


r2 = = =


since the common ratios are not equal they are not in GP


More from this chapter

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The first and the last terms of an AP are 17 and 350 respectively. If the common difference is 9, how many terms are there and what is their sum?

 5

Find the sum of first 51 terms of an AP whose second and third terms are 14 and 18 respectively.

 7

Show that a1, a2,…..an form an AP where is defined as below :

an = 3+4n


Also find the sum of the first 15 terms in each case.

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Show that a1, a2,…..an form an AP where is defined as below :

an = 9-5n


Also find the sum of the first 15 terms in each case.