# Find a if the coefficients of x2 and x3 in the expansion of (3 + ax)9 are equal.

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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