MathematicsFind the sum of the first 20 terms of the arithmetic series in which the 3rd term is 7 and 7th term is 2 more than three times its 3rd term.

Find the sum of the first 20 terms of the arithmetic series in which the 3rd term is 7 and 7th term is 2 more than three times its 3rd term.


  1. A
    740
  2. B
    703
  3. C
    679
  4. D
    639 

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    Solution:

    Let a and d be the first term and the common difference of the AP.
    a3=a+2d=7
    a7=a+6d=2+3a3=2+3×7=23
    4d=23-7=16
    d=4
    a=-1
    Sn=n2[2a+(n-1)d]=2022×-1+19×4=10×76-2=740
      
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