Question
If x+y = 1 ,then ∑ r nCrxryn-requals nx....How??
Joshi sir comment
∑ r nCrxryn-r
= ∑ r nCrxr(1-x)n-r
= ∑ r n!/r!(n-r)! xr(1-x)n-r
= n (n-1)!/(r-1)!(n-r)! xr(1-x)n-r
= n (n-1)C(r-1) xr(1-x)n-r
= n (n-1)Cr xr+1(1-x)n-r-1 let r = r+1
= nx (n-1)Cr xr(1-x)n-r-1
now solve