Upgrade to access all content for this subject

Let $x$ be a nonzero number. Which of the following is equal to:$$\sum_{k=0}^{n-1}(k+1)\binom{n}{k+1}x^{k}$$

$(k+1)(1+x)^{n-1}$

$n(1+x)^{n-1}$

$n(1+x)^n$

$(1+x)^n-x^n$