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For some n 10, let the coefficients of the 5 th, 6 th and 7 th terms in the binomial expansion of (1+x)n+4 be in A.P. Then the largest coefficient in the expansion of (1+x)n+4 is:
(2) 35
T5,T6,T7 have coefficients N4, N5, N6 with N=n+4; A.P. 2 N5= N4+ N6
2= 5N-4+ N-56 N2-21N+98=0 N=7 or 14
n 10 N=7, i.e. (1+x)7
Greatest coefficient (two middle terms): 73= 74=35
Answer: (2)
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