Answers

2014-05-20T16:10:12+05:30
n C_{r} = n C_{s}  then n = r + s

n C_{4} = n C_{6}  then n = 6 + 4 = 10

12 C_{n} = 12 C_{10}

=  \frac{12!}{10!(12-10)!}

= \frac{12!}{10!(2)!}

=\frac{12*11}{2}

= 66
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2014-05-20T17:32:40+05:30
Given  nc4 = nc6  
let us consider ncr=nct
this is possible only when r=t   or r+t = n 
but from the question 4 is not equal to 6.
so we can consider 4+6 =n
 therefore   n= 10 
Now we have to find 12cn
             = 12c10
              =66
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