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SSC CGL Model Paper 6

Question : 64 of 100

Marks: +1, -0

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 = \frac{n}{2}[2a + (n-1)d]

S_{11} = \frac{11}{2}[2 \times (-9) + 11 - 1 \times 4]

S_{11} = \frac{11}{2}[-18+40] = \frac{11}{2}[22]

S_{11} = 121

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