SSC CGL Model Paper 6

Question : 64 of 100

Marks: +1, -0

What is the sum of the first 11 terms of an arithmetic progression if the 3rd term is -1 and the 8th term is 19?

Solution:

Let first term be ‘a' and common difference be ‘d' in A.P.
According to the question,

T_n = a + ( n - 1) d

-1 = a + (3 - 1 ) d

-1 = a + 2d

,..(i)

19 = a + ( 8 -1) d

19 = a + 7d

...(ii)
On solving Eqs. (i) and (ii)

a = -9 , d =4

S_n=n/2[2a + ( n - 1) d ]

S_11=11/2[2×(-9)+11-1 ×4]

S_11=11/2[-18+40]=11/2[22]

S_11 =121

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