Câu 5 :
Ta có : \(\dfrac{1}{101}>\dfrac{1}{150};\dfrac{1}{102}>\dfrac{1}{150}\)
\(....;\dfrac{1}{149}>\dfrac{1}{150};\dfrac{1}{150}=\dfrac{1}{150}\)
\(\Leftrightarrow\dfrac{1}{101}+\dfrac{1}{102}+...+\dfrac{1}{150}>\dfrac{1}{150}+...+\dfrac{1}{150}\left(50SH\right)=\dfrac{1}{3}\)
Ta có :
\(\dfrac{1}{151}>\dfrac{1}{200};\dfrac{1}{152}>\dfrac{1}{200};...;\dfrac{1}{200}=\dfrac{1}{200}\)
\(\Leftrightarrow\dfrac{1}{151}+\dfrac{1}{152}+...+\dfrac{1}{200}>\dfrac{1}{200}+\dfrac{1}{200}+...+\dfrac{1}{200}\left(50SH\right)=\dfrac{1}{4}\)
\(\Leftrightarrow\dfrac{1}{101}+...+\dfrac{1}{200}>\dfrac{1}{3}+\dfrac{1}{4}=\dfrac{7}{12}\)
\(\Leftrightarrow A>\dfrac{7}{12}\)
Câu 4:
a: OB=OA+AB=5+2=7(cm)
b: C là trung điểm AB nên CA=CB
=>CD+DA=CB
=>CD=(BD-AD)/2