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If $^{n} C_{r} :^{n} C_{r + 1} :^{n} C_{r + 2} = 1 : 2 : 3$ then $r$ is equal to

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Correct option is B

21= nCr+1 nCr=n−rr+1and 32=n−r−1r+2⇒32=2(r+1)−1r+2 [∵n−r=2(r+1)]⇒3r+6=4r+2⇒r=4.

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If nCr:nCr+1:nCr+2=1:2:3 then r is equal to

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If $^{n} C_{r} :^{n} C_{r + 1} :^{n} C_{r + 2} = 1 : 2 : 3$ then $r$ is equal to