Đáp án+Giải thích các bước giải:
$A=\dfrac{\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...+\dfrac{1}{20}}{\dfrac{19}{1}+\dfrac{18}{2}+\dfrac{17}{3}+...+\dfrac{1}{19}}\\=\dfrac{\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...+\dfrac{1}{20}}{1+\left(\dfrac{18}{2}+1\right)+\left(\dfrac{17}{3}+1\right)+...+\left(\dfrac{1}{19}+1\right)}\\=\\\dfrac{\dfrac{1}{2}+\dfrac{1}{3}+..+\dfrac{1}{20}}{1+\dfrac{20}{2}+\dfrac{20}{3}+...+\dfrac{20}{19}}\\=\dfrac{\dfrac{1}{2}+\dfrac{1}{3}+..+\dfrac{1}{20}}{20.\left(\dfrac{1}{2}+\dfrac{1}{3}+...+\dfrac{1}{19}+\dfrac{1}{20}\right)}\\=\dfrac{1}{20}$
