Question

40(i) The sum of first 8 terms of an AP is 100 and some of first 19 terms is 551. Find AP

Answer 1

Answer:

hi mate

S8= 100, S19= 551

Sn= n/2(2a+(n-1)d)

S8= 8/2(2a+(8-1)d)

S8= 4(2a+7d)

100= 8a+28d...........................(1)

S19= 19/2(2a+18d)

551= 19a+ 171d ....................(2)

From (1) & (2)

152a+532d= 1900......................(3)

  152 a+ 1368d= 4408..................(4)

Using elimination method ie solving eq (3) and (4)

  ie 152 a +  532d  = 1900

      152 a +1368d  = 4408

-------------------------------------------

                 -836 d  = - 2508

d= 3

Therefore a=  2

hope it helps you