Question
Sun December 02, 2012 By:

THE SUM OF FIRST 8 TERM OF AN AP IS 100 AND THE SUM OF ITS FIRST 19 TERM IS 551. FIND THE AP,

Expert Reply
Mon December 03, 2012

sum of n terms in AP can be given by (n/2)*[2a+(n-1)d]

sum of first 8 terms = 4*[2a+7d] =100

or 2a+7d = 25       (1)

sum of first 19 terms = (19/2)*[2a+18d] =551

or 2a+18d = 58      (2)

subtracting (2) - (1), we get

11d =33

d =3

2a+7*3= 25

a =2

hence the AP is 2,5,8,11...
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