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If ""^(n)P(r)=""^(n)P((r+1))and ""^(n)...

If ` ""^(n)P_(r)=""^(n)P_((r+1))`and `""^(n)C_(r) = ""^(n)C_(r-1),` then (n,r) =

A

(8,9)

B

(6,7)

C

(4,5)

D

(3,2)

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

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

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

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  5. ""^(2n)C(n+1)+2. ""^(2n)C(n) + ""^(2n) C(n-1) =

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

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

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

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

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

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

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  13. ""^(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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  15. In a football championship there were played 153 matches. Every two te...

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

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

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

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