Câu hỏi 32Nếu $x_{k+1}=x_k+\dfrac12$ cho $k=1,$ $2,$ $\ldots,$ $n-1$ và $x_1=1,$ tìm $x_1+x_2+\cdots+x_n.$$\dfrac{n+1}{2}$$\dfrac{n+3}{2}$$\dfrac{n^2-1}{2}$$\dfrac{n^2+n}{4}$$\dfrac{n^2+3n}{4}$