Solution:
Coefficient of ,that of and that of . According to the condition, .Hence,
or
After solving, we get n = 7 or 14.
If the coefficients of 5th, 6th and 7th terms in the expansion of are in A.P., then n =
Coefficient of ,that of and that of . According to the condition, .Hence,
or
After solving, we get n = 7 or 14.