S= (\(\dfrac{1}{31}\)+...+\(\dfrac{1}{40}\)(\(\dfrac{1}{41}\) + ... + \(\dfrac{1}{50}\)) + (\(\dfrac{1}{51}\) + ... + \(\dfrac{1}{60}\)) < \(\dfrac{10}{31}\) + \(\dfrac{10}{41}\) + \(\dfrac{10}{51}\) < \(\dfrac{10}{30}\) + \(\dfrac{10}{40}\)+ \(\dfrac{10}{50}\) = \(\dfrac{1}{3}\) +\(\dfrac{1}{4}\) + \(\dfrac{1}{5}\)= \(\dfrac{7}{12}\)  +\(\dfrac{1}{5}\)< \(\dfrac{3}{5}\)+ \(\dfrac{1}{5}\)=\(\dfrac{4}{5}\)

Tương tự:
S > \(\dfrac{10}{40}\) + \(\dfrac{10}{50}\) + \(\dfrac{10}{60}\)= \(\dfrac{1}{4}\) + \(\dfrac{1}{5}\) + \(\dfrac{1}{6}\)= \(\dfrac{5}{12}\)+ \(\dfrac{1}{5}\)> \(\dfrac{2}{5}\) + \(\dfrac{1}{5}\) = \(\dfrac{3}{5}\)
=> \(\dfrac{3}{5}\) < S < 4 / \(\dfrac{4}{5}\)

1.a) Điểm O thuộc những đoạn thẳng: CD, AB

b) Điểm O là trung điểm của đoạn thẳng AB 

2. a) góc xOy= 40 độ