Question

If 21C2r+1=21Cr-4,then find r​

Answer 1

Answer:

Given 20Cr = 20C r−10

We Know

nC r = nC n−r

⇒r+(n−r)=n

⇒r+r−10=20

⇒2r=30

⇒r=15

⇒18Cr = 18 C 15

⇒18! / 15!×3!

⇒3×2×1/18×17×16

⇒48×17

⇒816

Hence the correct answer is 816

Step-by-step explanation:

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Answer 2

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Refer to the above given attachment ☝

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