If `^n+1C_(r+1)dot^n C_rdot^(n-1)C_(r-1)=11 :6:3,` then `n r=` `20` b. `30` c. `40` d. `50`
If `^n+1C_(r+1)dot^n C_rdot^(n-1)C_(r-1)=11 :6:3,`
then `n r=`
`20`
b. `30`
c. `40`
d. `50`
A. `20`
B. `30`
C. `40`
D. `50`
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1 Answers
Correct Answer - D
Given,
`(.^(n+1)C_(r+1))/(.^(n)C_(r))= 11/6` or `((n+1)/(r+1)xx.^(n)C_(r))/(.^(n)C_(r)) = 11/6`
or `6n+6 = 11 r + 11` or `6n-11r = 5" "(1)`
Also,
`(.^(n)C_(r))/(.^(n-1)C_(r-1)) = 6/3` or `(n/rxx.^(n-1)C_(r-1))/(.^(n-1)C_(r-1)) = 6/3` or `n = 2r " " (2)`
From (1) and (2), `r = 5` and `n = 10`,
`:. nr = 50`
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