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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 Binomial Theorem, Binomial Formula, Binomial Coefficients and Binomial Expansion in this site.
7-th term is 
.
6-th term is 
.
5-th term is 
.
= 7; 
= 
= 21; = 
= 35.
This AP is 35, 21, 7. Or 7, 21, 35 from the other end.
