Class XI Chapter 9 – Sequences and Series Maths ______________________________________________________________________________ Let the sum of n terms of the given A.P. be 25 . It is known that, n Sn 2a n 1 d 2 Where n number of terms, a first term, and d common difference Here, **11** **11**12 1 d 6 2 2 2 Therefore, we obtain a 6 n 1 25 2 6 n 1 2 2 n 1 50 n 12 2 2 25 n 50 n 2 2 100 n 25 n 2 n n 25 100 0 2 n n n 5 20 100 0 n n n 5 20 5 0 n 20 or 5 Question 5: In an A.P., if pq 1 , where Solution 5: th p term is 1/q and p q . th q It is known that the general term of an A.P. is According to the given information, th 1 p term ap a p 1 d ...... 1 q th 1 q term aq a q 1 d ...... 2 p Subtracting (2) from (1), we obtain term is 1/q, prove that the sum of first a a n 1 d n pq terms is 1/2 Printed from Vedantu.com. Register now to book a Free LIVE Online trial session with a Top tutor.

Class XI Chapter 9 – Sequences and Series Maths ______________________________________________________________________________ 1 1 p d q d q p 1 1 p q p 1 q 1 d pq p q d 1 d pq p q pq Putting the value of d in (1), we obtain 1 1 a p1 pq q 1 1 1 1 a q q pq pq pq S pq 2a pq 1 d 2 pq 2 pq 1 1 2 pq pq 1 1 1 2 pq 1 1 1 1 pq 1 pq 2 2 2 2 1 1 2 pq Thus, the sum of first pq terms of the A.P. is 1 1 2 pq . Question 6: If the sum of a certain number of terms of the A.P. 25, 22, 19, …… is **11**6. Find the last term Solution 6: Let the sum of n n Sn 2a n 1 d 2 terms of the given A.P. be **11**6. Here, a 25 and d 22 25 3 Printed from Vedantu.com. Register now to book a Free LIVE Online trial session with a Top tutor.

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