Question

the sum of the first 8terms of an ap is 100 and the sum of its first 19 terms is 551 find the ap​

Answer 1

Ans- S8 = 100

S19= 551

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

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

100=4{2a+7d}

2a+7d=25

S19= 19/2 {2a+(19-1)d}

551×2/19=2a+18d

2a+18d=58

By Elimination Method

2a+18d=58

(-) 2a+ 7d=25

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11d=33

d=3

2a+7d= 25

2a+7(3)=25

2a=25-21

a=4/2

a = 2

AP = 2,5,8,11,14....…..