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CLASS_11_MATHS_SOLUTIONS_NCERT

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Class XI Chapter 9 – Sequences and Series Maths<br />

______________________________________________________________________________<br />

1 1<br />

p d q d <br />

q p<br />

1 1<br />

p<br />

q<br />

p 1 q 1<br />

d <br />

pq<br />

<br />

<br />

p q d <br />

1<br />

d<br />

<br />

pq<br />

p<br />

q<br />

pq<br />

Putting the value of d in (1), we obtain<br />

1 1<br />

a p1<br />

<br />

pq q<br />

1 1 1 1<br />

a <br />

q q pq pq<br />

pq<br />

S<br />

pq<br />

2a pq 1<br />

d <br />

2<br />

<br />

<br />

pq 2 pq 1<br />

1 <br />

<br />

2<br />

<br />

pq pq<br />

<br />

<br />

<br />

1<br />

1 1<br />

2 pq <br />

1 1 1 1<br />

pq 1 pq <br />

2 2 2 2<br />

1<br />

1<br />

2 pq <br />

<br />

<br />

Thus, the sum of first pq terms of the A.P. is<br />

1<br />

1<br />

2 pq <br />

<br />

.<br />

Question 6:<br />

If the sum of a certain number of terms of the A.P. 25, 22, 19, …… is <strong>11</strong>6.<br />

Find the last term<br />

Solution 6:<br />

Let the sum of n<br />

n<br />

Sn<br />

2a n 1<br />

d <br />

2<br />

<br />

<br />

terms of the given A.P. be <strong>11</strong>6.<br />

Here, a 25 and d 22 25 3<br />

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