Q. 24.2( 31 Votes )
Find a if the coefficients of x2 and x3 in the expansion of (3 + ax)9 are equal.
Answer :
We know that-
General term of expansion (a+b)n is
For (3+ax)9
Putting a = 3, b = ax & n = 9
General term of (3+ax)9 is
Since we need to find the coefficients of x2 and x3, therefore
For r = 2
Thus, the coefficient of x2 = 
For r = 3
Thus, the coefficient of x3 = 
Given that-
Coefficient of x2 = Coefficient of x3
∴ a = 9/7
Hence, a = 9/7
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