Question

if 20Cr=20Cr+4then is equal to.....​

Answer 1

r + r + 4 = 20 [∵nCx=nCy ⇒n=x+y or x=y]

⇒2r + 4 = 20⇒ 2r = 16⇒ r = 8

Now,rC3 = 8C3

8C3 = 8!3! 5! = 8×7×63×2×1 = 56

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