Q. 24.2( 166 Votes )

The sum of the third and the seventh terms of an AP is 6 and their product is 8. Find the sum of first sixteen terms of the AP.

Answer :

To Find: S16 
Given: a3 + a7 = 6
           a3 x a7 = 8

nth term of an AP is given by the formula
an = a + (n - 1) d
where, an = nth term 
n = number of term
d = common difference
So now its given that sum of third and seventh term is 6, thus we need to find 3rd and 7th term first,
a3 = a + 2d


a7 = a + 6d


As per question;


a3 + a7 = 6

So now,

a + 2d + a + 6d = 6


2a + 8d = 6


a + 4d = 3


a = 3 – 4d    ....................eq (i)


Similarly,

Product of third and seventh term is given as 8. So,

(a + 2d)(a + 6d) = 8


a2 + 6ad + 2ad + 12d2 = 8  ..........................eq(ii)


Substituting the value of a in equation (ii), we get;


(3 – 4d)2 + 8(3 – 4d)d + 12d2 = 8


9 – 24d + 16d2 + 24d – 32d2 + 12d2 = 8


9 – 4d2 = 8


2d = 1


d = ± 1/2


Using the value of d in equation (1), we get;


a = 3 – 4d



Or, a = 3 – 2 = 1


Sum of first 16 terms is calculated as follows:




S16 8[ 2 + (15/2)]


= 4 x 19


S16 = 76

Thus, sum of first 16 terms of this AP is 76.
Now by taking d = -1/2, we get,
a = 3 - 4 (-1/2)
a = 3 + 2 = 5

S = 8[10 - 15/2]
S = 4[20 - 15]
S = 4[5] = 20
So, another possible value of sum is 20.

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