1.

If`C_0,C_1,C_2..C_n` denote the coefficients in the binomial expansion of `(1 +x)^n`, then `C_0 + 2.C_1 +3.C_2+. (n+1) C_n`A. `n2^(n)`B. `n2^(n-1)`C. `2^(n-1)`D. `(n-1)2^(n-1)`

Answer» Correct Answer - b
We have,
`C_(1) + 2. C_(2) + 3. C_(3) +...+n C_(n)`
`= sum_(r=1)^(n) r. C_(r)`
`= sum_(r=1)^(n) r.""^(n) C_(r)" "[because C_(r) = ""^(n) C_(r)] `
`= sum_(r=1)^(n) r.(n)/(r) ""^(n-1)C_(r) [because ""^(n)C_(r) = (n)/(r)""^(n-1)C_(r-1) ]`
`= n sum_(r=1)^(n) (n)/(r) ""^(n-1)C_(r-1) `
`=( ""^(n-1)C_(0)+ ""^(n-1)C_(1) + ...+ ""^(n-1)C_(2)+...+""^(n-1)C_(n-1)) = n. 2^(n-1)`.


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