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Find r if P (20,r)=13.P(20,r-1)....

Find r if `P (20,r)=13.P(20,r-1).`

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`""^20P_r=13xx""^20P_(r-1)`
`or,(20!)/((20!-r)!)=13xx(20!)/((20-r+1)!)`
`or,(20-r)! =((21-r)(20-r)!)/(13)`
`or,21-r=13`
`or,r=8`
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MBD PUBLICATION-PERMUTATIONS AND COMBINATIONS-QUESTION BANK
  1. Compute the 2^3!

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  2. Show that 2.6.10…….to n factors =((2n)!)/(n!)

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  3. Find r if P (20,r)=13.P(20,r-1).

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  4. Find n"if" P (n,4)=12.P(n,2)

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  5. IfP(n-1,3):P(n+1,3)=5:12,"Find "n.

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  6. Find m and n if P(m+n,2)=56,P(m-n,2)=12

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  7. Show that P(n,n)=P(n,n-1)"For all positive integers."

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  8. Show that P(m,1)+P (n,1)=P(M+n,1)for all positive integers m, n.

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  19. Compute the ^12C3

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  20. Compute the ^15C12

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