Question

2. If the median term of a 7 parallel series is 21, find the sum of all the terms in that series.

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Answer 1

Here is ur answer

Let the three terms of A.P are a,a+d,a+2d.

Now,As per the question , 

a+a+d+a+2d=3a+3d=21

 i.e. a+d=7 or, d=7−a.

Now,a(a+2d)−(a+d)=6

a{a+2(7−a)}−7=6

a{a+14−2a}=13

a{14−a}=13

14a−a2=13

a2−14a+13=0

By factorising,we get

a=13,1

d=7−a 

For a=13,1d=7−13,7−1=−6,6 respectively.

Now, there are two AP's a,a+d,a+2d for two values of a and d

13,7,1  and 1,7,13