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IF ""^(n) P(r)=720,""^(n)C(r)=120then ...

IF ` ""^(n) P_(r)=720,""^(n)C_(r)=120`then `(n,r)`=

A

(7,4)

B

(6,2)

C

(8,4)

D

(10,3)

Text Solution

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The correct Answer is:
D
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AAKASH SERIES-PERMUTATIONS & COMBINATIONS-EXERCISE-II
  1. sum(r=0)^(n)(P(r))/(r!) is equal to

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  2. If ""^(n)P(r)=""^(n)P((r+1))and ""^(n)C(r) = ""^(n)C(r-1), then (n,...

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  3. IF ""^(n) P(r)=720,""^(n)C(r)=120then (n,r)=

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  4. ""^(2n)C(n+1)+2. ""^(2n)C(n) + ""^(2n) C(n-1) =

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  5. the value of ""^50C(4) + underset( r=1) overset( 6 ) sum ""^(56...

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  6. sum( r=0 )^ ( 10) ( 40 -r)C (5)=

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  7. If ""^(n-1)C(3)+""^(n-1)C(4)gt""^(n)C(3), then

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  8. If ""^(n)C(4),""^(n)C(5),""^(n)C(6) are in A.P., then the value of n i...

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  9. If ""^(n)C(r-1)=36,""^(n)C(r)=84 and ""^(n)C(r+1)=126, then r is equal...

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  10. The value of 2^(n)n!(1.3.5.....(2n-1)) is

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  11. If a(n)=sum(r=0)^(n)(1)/(""^(n)C(r)) then sum(r=0)^(n)(r)/(""^(n)C(r))...

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  12. ""^(n)C(r)+4.""^(n)C(r-1)+6.""^(n)C(r-2)+4.""^(n)C(r-3)+""^(n)C(r-4) i...

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  13. The sum sum(i=0)^(m)((10)/(i))((20)/(m-i)) is maximum when m is

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  14. In a football championship there were played 153 matches. Every two te...

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  15. From a group of persons the number of ways of selecting 5 persons is e...

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  16. In a chess tournament where the participants were to play one game wit...

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  17. A teacher takes 3 children from her class to the zoo at a time as ofte...

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  18. From 0 to 9, four digited numbers can be formed such that the digits a...

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  19. The total number of 4 digit numbers in which the digits are in descend...

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  20. There are 12 balls numbered from 1 to 12. The number of ways in which ...

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