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Determine the coefficient of x2 in the expansion of \...

Determine the coefficient of x2 in the expansion of (a+3x)6

A.

18a2

B.

45a4

C.

135a4

D.

1215a2

Correct answer is C

(a+3x)6.

The coefficient of x2 is:

6C4(a)62(3x)2=6!(64)!4!(a4)(9x2)

15×a4×9=135a4