Question

If 12th term of an AP is -12 and sum of first 4 terms is 24, what is the sum of first ten terms ?

Answer 1

nth term = a + (n-1)d
Sn = n/2{a + (n-1)d}
12th term = a + 11d = -12 ........ (i)
S4 = 4/2{a + 3d) = 24
               a + 3d = 12 ....... (ii)

From (i) a + 11d = -12
               a = -11d - 12  Substitute this in (ii)
-11d - 12 + 3d = 12
                   -8d = 24
                       d = -3

a = -11d -12 = -11x-3 - 12 = 21

S10 = 10/2 {21 + (10-1)X-3}
         = 5 (21 -27)
         = 5 X -6
          = -30

Answer 2

Answer:

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