Question

If nc2 = 105 find n.............................

Answer 1

nc2=105

According to formula

ncr=n!/(n-r)!*r!

n!/(n-2)!*2!=105

n*(n-1)*(n-2)!/(n-2)!*2!=105

n*(n-1)/2*1=105

n*(n-1)/2=105

n*(n-1)=210

n^2-n=210

n^2-n-210=0

n^2-15n+14n-210=0

n(n-15)+14(n-15)=0

(n+14)+(n-15)=0

n+14=0 | n-15=0

n=-14 ( Rejected because roots cannot be negative) | n=15

So n is 15

I hope it will help you