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Questions  

 The coefficient of x5 in the expansion of (1+x)21+(1+x)22++(1+x)30 is 

a
51C5
b
9C5
c
31C6−21C 6
d
30C5+ 20C 5

detailed solution

Correct option is C

coefficientof x5 in (1+x)21+(1+x)22+…+(1+x)30 =coefficientof x5 in (1+x)21(1+x)10-1(1+x)-1 =coefficient of x6 in (1+x)31-(1+x)21

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