Mathematics, 29.06.2019 22:00 discolldiane7351
The sum of the first n terms of an a. p. is 2n and the sum of the first 2n terms is n. find the sum of the first 4n terms.
Answers: 1
![S_n=\frac{n}{2}[2a+(n-1)d]](/tpl/images/0032/5459/b603c.png)
![\frac{n}{2}[2a+(n-1)d]=2n](/tpl/images/0032/5459/6b2b7.png)
             ..... (1)![\frac{2n}{2}[2a+(2n-1)d]=n](/tpl/images/0032/5459/8fffa.png)
             ..... (2)
![S_{4n}=\frac{4n}{2}[2a+(4n-1)d]](/tpl/images/0032/5459/2df69.png)
![S_{4n}=2n[2a+(4n-1)d]](/tpl/images/0032/5459/13810.png)
![S_{4n}=2n[2(\frac{1}{2}(7-\frac{3}{n}))+(4n-1)(-\frac{3}{n})]](/tpl/images/0032/5459/ab54b.png)
![S_{4n}=2n[7-\frac{3}{n}-12+\frac{3}{n}]](/tpl/images/0032/5459/8d596.png)
![S_{4n}=2n[-5]](/tpl/images/0032/5459/a584a.png)
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The sum of the first n terms of an a. p. is 2n and the sum of the first 2n terms is n. find the sum...
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