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Question

The coefficient of x3 in the expansion of (1+3x)21−2x

will be


A

8

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B

32

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C

50

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D

None of these

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Solution

The correct option is C

50


= (1+3x)2(12x)1

= (1+3x)2(1 + 2x + 1.22.1(2x)2+.......)

= (1+6x+9x2)(1+2x+4x2+8x3+........)

Therefore coefficient of x3 is (8 + 24 + 18) = 50.


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