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Questions  

The coefficient of x7 in the expansion of 1+11!x+12!x2+13!x3+14!x4+15!x52is

a
27! 8C3
b
2105!
c
415
d
720!

detailed solution

Correct option is A

Required coefficient is12!5!+13!4!+14!3!+15!2!=17! 7C2+7C3+7C4+7C5=27! 7C2+7C3=27! 8C3

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