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Questions  

Coefficient of x4 in the expansion of 1+3x+2x26is:

a
240
b
735
c
820
d

detailed solution

Correct option is B

The general term in the expansion of 1+3x+2x26is                                                 6!a!b!c!1a(3x)b2x2c where a,b,c≥0 and a+b+c=6                                                  =6!3b26−a−ba!b!(6−a−b)!xb+2a+2b−12 for coefficient of x4, set 2a+3b-12=4Now 2a+3b=16 and a+b+c=6⇒(a,b,c)=(2,4,0)Thus, required coefficient is 6!34202!4!=735

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