Question

In an A.P. if the 12th term is -13 and the sum of its first four terms is 24. Find
the sum of its first 20 terms.
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Answer 1

Answer:

Step-by-step explanation:

T12=a+11d

-13 = a + 11d......................(1)

then S4 = 24

T1+T2+T3+T4 = 24

(a)+(a+d)+(a+2d)+(a+3d) = 24

4a+6d = 24

2a + 3d = 12....................(2)

solving 1 and 2

a= 9 and d = -2

and S20 = 10 x (2(18) + 19x-2)

              = 10 x ( 36 -38)

              = 10 (-2)

              = -20