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Let  be the terms of a sequence and let

Case I:

If  are in A.P., then  is quadratic in ‘’. If  are in A.P., then  is cubic in

Case II:

If  are not in A.P., but in G.P., then , where  is the common ratio of the G.P.  and  Again, ifare not in G.P. but  are in G.P., then  is of form  and  is the common ratio of the G.P.  and

 

The sum of 20 terms of the series  is

 

a)

4010

b)

3860

c)

4240

d)

None of these


Question ID - 51133 | Toppr Answer

Let  be the terms of a sequence and let

Case I:

If  are in A.P., then  is quadratic in ‘’. If  are in A.P., then  is cubic in

Case II:

If  are not in A.P., but in G.P., then , where  is the common ratio of the G.P.  and  Again, ifare not in G.P. but  are in G.P., then  is of form  and  is the common ratio of the G.P.  and

 

The sum of 20 terms of the series  is

 

a)

4010

b)

3860

c)

4240

d)

None of these

1 Answer - 5876 Votes

3537

Answer Key : (c) -

(c)

Clearly here the differences between the successive terms are

 i.e…4, 7, 10, … which are in A.P.

Thus, we have

Solving, we get . Hence,

=4240



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