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CLASS_11_MATHS_SOLUTIONS_NCERT

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

______________________________________________________________________________<br />

Let the sum of n terms of the given A.P. be 25 .<br />

It is known that,<br />

n<br />

Sn<br />

2a n 1<br />

d <br />

2<br />

<br />

<br />

Where n number of terms, a first term, and d common difference<br />

Here,<br />

<strong>11</strong> <strong>11</strong>12 1<br />

d 6 <br />

2 2 2<br />

Therefore, we obtain<br />

a 6<br />

n <br />

1<br />

<br />

25 2 6 n<br />

1<br />

2 <br />

2 <br />

<br />

<br />

n 1<br />

50 n <br />

<br />

12 <br />

<br />

2 2 <br />

<br />

25 n<br />

50<br />

n <br />

<br />

<br />

<br />

2 2<br />

<br />

100 n 25<br />

n<br />

2<br />

n n <br />

<br />

25 100 0<br />

2<br />

n n n <br />

5 20 100 0<br />

n<br />

<br />

n n 5 20 5 0<br />

n<br />

<br />

20 or 5<br />

<br />

Question 5:<br />

In an A.P., if<br />

<br />

pq 1<br />

<br />

, where<br />

Solution 5:<br />

th<br />

p<br />

term is 1/q and<br />

p<br />

q<br />

.<br />

th<br />

q<br />

It is known that the general term of an A.P. is<br />

According to the given information,<br />

th<br />

1<br />

p term ap<br />

a p 1 d ...... 1<br />

q<br />

<br />

th<br />

1<br />

q term aq<br />

a q 1 d ...... 2<br />

p<br />

<br />

Subtracting (2) from (1), we obtain<br />

term is 1/q, prove that the sum of first<br />

a a n 1<br />

d<br />

n<br />

<br />

<br />

pq<br />

terms is 1/2<br />

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