If 1+(1+x)+(1+x)2+(1+x)3+……+(1+x)n=∑nK=0aKXK, then which of the following is true?
A
an−2=n(n−1)2
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B
a29−a28=n+1C10(n+1C10−n+1C8)
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C
aK=nCK
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D
∑nK=0aK=2n+1−1
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Solution
The correct option is D∑nK=0aK=2n+1−1 1+(1+x)+(1+x)2+(1+x)3+……+(1+x)n=∑nK=0aKXK So the coefficient of xK in (1+x)n+1−1x=aK=n+1CK+1 an−2=n+1Cn−1=n(n+1)2 a29−a28=n+2C10(n+1C10−n+1C9) ∑nK=0aK=n+1C1+n+1C2+……+n+1Cn+1=2n+1−1