the sum of first 8terms of an ap is 100 and sum of first 19 terms is 551.find ap
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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...
I hope this help you