Question 1014396
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The coefficient of 5th 6th and 7th terms in the expansion ( 1+a)^7 in ascending powers of a forms the arithmetic progressions. Find the A.P 
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On binomial expansion see the lesson <A HREF=http://www.algebra.com/algebra/homework/Permutations/Binomial-Theorem.lesson>Binomial Theorem, Binomial Formula, Binomial Coefficients and Binomial Expansion</A> in this site.

7-th term is {{{C[7]^6*a^6}}}.

6-th term is {{{C[7]^5*a^5}}}.

5-th term is {{{C[7]^4*a^4}}}.

{{{C[7]^6}}} = 7;  {{{C[7]^5}}} = {{{(7*6)/2}}} = 21;  {{{C[7]^5}}} = {{{(7*6*5)/(1*2*3)}}} = 35.

This AP is  35,  21,  7.   Or 7,  21,  35 from the other end.
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