Bài 12:
\(\dfrac{3}{14}+\dfrac{3}{14}+\dfrac{3}{14}+\dfrac{3}{14}+\dfrac{3}{14}< \dfrac{3}{10}+\dfrac{3}{11}+\dfrac{3}{12}+\dfrac{3}{13}+\dfrac{3}{14}< \dfrac{3}{10}+\dfrac{3}{10}+\dfrac{3}{10}+\dfrac{3}{10}+\dfrac{3}{10}\)\(\Rightarrow\dfrac{15}{14}< S< \dfrac{3}{2}\).
\(\Rightarrow1< \dfrac{15}{14}< S< \dfrac{3}{2}< 2\)
Bài 13:
\(\dfrac{1}{5}+\dfrac{1}{13}+\dfrac{1}{14}+\dfrac{1}{15}+\dfrac{1}{61}+\dfrac{1}{62}+\dfrac{1}{63}< \dfrac{1}{14}+\dfrac{1}{14}+\dfrac{1}{14}+\dfrac{1}{14}+\dfrac{1}{14}+\dfrac{1}{14}+\dfrac{1}{14}\)
\(\Rightarrow S< \dfrac{1}{2}\)
