Question

Tính  A/B biết rằng:                                                                                                 A= 1/1*300 + 1/2*301 +...+ 1/101*400                                                                B= 1/1*102 + 1/2*103 +...+ 1/298*399 + 1/299*400

Answer 1

\(A=\dfrac{1}{1.300}+\dfrac{1}{2.301}+...+\dfrac{1}{101.400}\)

\(\Rightarrow299A=\dfrac{299}{1.300}+\dfrac{299}{2.301}+...+\dfrac{299}{101.400}=1-\dfrac{1}{300}+\dfrac{1}{2}-\dfrac{1}{301}+...+\dfrac{1}{101}-\dfrac{1}{400}=M\)

\(\Rightarrow A=\dfrac{M}{299}\left(1\right)\)

Ta lại có:

\(B=\dfrac{1}{1.102}+\dfrac{1}{2.103}+...+\dfrac{1}{298.399}+\dfrac{1}{299.400}\)

\(\Rightarrow101B=\dfrac{101}{1.102}+\dfrac{101}{2.103}+...+\dfrac{101}{399.400}=1-\dfrac{1}{102}+\dfrac{1}{2}-\dfrac{1}{103}+...+\dfrac{1}{399}-\dfrac{1}{400}=1-\dfrac{1}{300}+\dfrac{1}{2}-\dfrac{1}{301}+...+\dfrac{1}{101}-\dfrac{1}{400}=M\)

\(\Rightarrow B=\dfrac{M}{101}\left(2\right)\)

Từ \(\left(1\right),\left(2\right)\Rightarrow\dfrac{A}{B}=\dfrac{M}{299}:\dfrac{M}{101}=\dfrac{101}{299}\)