Coefficients of Terms Equidistant from Beginning and End
The coefficie...
Question
The coefficient of xm in (1+x)m+(1+x)m+1+⋯+(1+x)n, where m,n∈N and m<n, is
A
nCm
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B
nCm+1
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C
n+1Cm+1
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D
n+2Cm+2
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Solution
The correct option is Cn+1Cm+1 The coefficient of xm in (1+x)m+(1+x)m+1+(1+x)m+2+⋯+(1+x)n=mCm+m+1Cm+m+2Cm+⋯+nCm=m+1Cm+1+m+1Cm+m+2Cm+⋯+nCm(∵mCm=m+1Cm+1=1)=n+1Cm+1(∵nCr+nCr+1=n+1Cr+1)