Đáp án: $\dfrac{1}{50}$
Giải thích các bước giải:
$P= \dfrac{1}{49}+\dfrac{2}{48}+\dfrac{3}{47}+...+\dfrac{48}{2}+49$
$= \left ( 1+\dfrac{1}{49} \right )+\left ( 1+\dfrac{2}{48} \right )+\left ( 1+\dfrac{3}{47} \right )+...+\left (1+\dfrac{48}{2} \right )+1$
$= \dfrac{50}{49}+\dfrac{50}{48}+\dfrac{50}{47}+...+\dfrac{50}{2}+\dfrac{50}{50}$
$= 50\left ( \dfrac{1}{49}+\dfrac{1}{48}+...+\dfrac{1}{2}+\dfrac{1}{50} \right )$
$= 50S$