Giải thích các bước giải:
$D=\dfrac{1}{2.3}+\dfrac{1}{3.4}+..+\dfrac{1}{19.20}$
$\to D=\dfrac{3-2}{2.3}+\dfrac{4-3}{3.4}+..+\dfrac{20-19}{19.20}$
$\to D=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+..+\dfrac{1}{19}-\dfrac{1}{20}$
$\to D=\dfrac{9}{20}$
$E=\dfrac{1}{99}-\dfrac{1}{99.98}-\dfrac{1}{98.97}-..-\dfrac{1}{3.2}-\dfrac{1}{2.1}$
$E=\dfrac{1}{99}-(\dfrac{1}{99.98}+\dfrac{1}{98.97}+..+\dfrac{1}{3.2}+\dfrac{1}{2.1})$
$\to E=\dfrac{1}{99}-(\dfrac{99-98}{99.98}+\dfrac{98-97}{98.97}+..+\dfrac{3-2}{3.2}+\dfrac{2-1}{2.1})$
$\to E=\dfrac{1}{99}-(\dfrac{1}{98}-\dfrac{1}{99}+\dfrac{1}{97}-\dfrac{1}{98}+..+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{1}-\dfrac{1}{2})$
$\to E=\dfrac{1}{99}-(1-\dfrac{1}{99})$
$\to E=\dfrac{2}{99}-1$
$\to E=\dfrac{-97}{99}$
