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B
n+m+2Cn
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C
n+m+3Cn+1
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D
none of these
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Solution
The correct option is An+m+1Cn+1 Given ∑mr=0n+rCn =nCn+n+1Cn+n+2Cn+n+3Cn.....n+mCn =n+1Cn+1+n+1Cn+n+2Cn+n+3Cn.....n+mCn (∵nCn=n+1Cn+1) =n+2Cn+1+n+2Cn+n+3Cn.....n+mCn (∵nCr+nCr−1=n+1Cr) =n+3Cn+1+n+3Cn+......n+mCn Continuing this , =n+mCn+1+n+mCn =n+m+1Cn+1