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CLASS_11_MATHS_SOLUTIONS_NCERT

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

______________________________________________________________________________<br />

2 1 1<br />

<br />

d c e<br />

<br />

...... 3<br />

It has to be proved that a,,<br />

c e are in G.P. i.e.,<br />

From (1), we obtain<br />

2b a c<br />

a<br />

c<br />

b<br />

<br />

2<br />

From (2), we obtain<br />

2<br />

c<br />

d <br />

b<br />

Substituting these values in (3), we obtain<br />

2b<br />

1 1<br />

<br />

2<br />

c c e<br />

<br />

2 a<br />

c 1 1<br />

<br />

2<br />

2c c e<br />

<br />

a c e c<br />

<br />

2<br />

c ce<br />

a c e c<br />

<br />

c e<br />

a ce e cc<br />

2<br />

ae ce ec c<br />

<br />

2<br />

c<br />

Thus,<br />

ae<br />

ac ,<br />

and e are in G.P.<br />

2<br />

c<br />

ae<br />

Question 21:<br />

Find the sum of the following series up to n terms:<br />

(i) 5 55 555 .... (ii) .6 .66 .666 ...<br />

<br />

Solution 21:<br />

(i) 555 555 ....<br />

Let<br />

S<br />

<br />

n<br />

5 55 555 ....to n terms<br />

5<br />

9 99 999 .....to n terms<br />

9<br />

<br />

<br />

2 3<br />

5 10 1 10 1 10 1 ...to n terms <br />

9 <br />

<br />

2 3<br />

5 10 10 10 to n terms 1 1 ...to n terms <br />

9 <br />

<br />

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