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Home / Doubts and Solutions/ICSE/Class 12 Science/Mathematics

Find the sum of the first 19 terms of an arithmetic progression, if the sum of the first, the third, the eighth, the twelfth, the seventeenth and the nineteenth terms is 555.

Asked by priyanshipandeypp98 24th October 2019, 7:33 PM
Answered by Expert
Answer:
Let a and d be the first term and common difference respectively
As per the question,
a+a+2d+a+7d+a+11d+a+16d+a+18d = 555
6a+54d=555
N o w comma space t h e space s u m space o f space f i r s t space 19 space t e r m s space i s space g i v e n space b y S subscript 19 equals 19 over 2 open parentheses 2 a plus 18 d close parentheses equals fraction numerator 19 over denominator 2 cross times 3 end fraction open parentheses 6 a plus 54 d close parentheses equals fraction numerator 19 over denominator 2 cross times 3 end fraction cross times 555 equals fraction numerator 19 cross times 185 over denominator 2 end fraction equals 3515 over 2
Answered by Expert 25th October 2019, 10:59 AM
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