Câu hỏi 12
Với $r=1,2,3,\ldots$, đặt $u_r=1+2+3+\cdots+r$. Tính
$$\frac1{1/u_1}+\frac2{1/u_1+1/u_2}+\frac3{1/u_1+1/u_2+1/u_3}+\cdots+\frac{100}{1/u_1+1/u_2+\cdots+1/u_{100}}.$$
$$\frac1{1/u_1}+\frac2{1/u_1+1/u_2}+\frac3{1/u_1+1/u_2+1/u_3}+\cdots+\frac{100}{1/u_1+1/u_2+\cdots+1/u_{100}}.$$