a) Ta có :

11 . ( y - 6) = 4.y +11

\(\Rightarrow11.y-11.6=4.y+11\)

\(\Rightarrow11.y-4.y=11+11.6\)

\(\Rightarrow y.\left(11-4\right)=11.\left(1+6\right)\)

\(\Rightarrow y.7=11.7\)

\(\Rightarrow y=11\)

Lần này không nhắc nhớ tick mik điii ạk

Ta có : P = \(\dfrac{5}{1.4}+\dfrac{5}{4.7}+\dfrac{5}{7.11}+...+\dfrac{5}{2017.2020}\)

\(\Rightarrow P=5\left(\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.11}+...+\dfrac{1}{2017.2020}\right)\)

\(\Rightarrow\) \(P=5.\dfrac{3}{3}\left(\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.11}+...+\dfrac{1}{2017.2020}\right)\)

\(\Rightarrow P=5.\dfrac{1}{3}\left(\dfrac{1}{1}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+...+\dfrac{1}{2017}-\dfrac{1}{2020}\right)\)

\(\Rightarrow P=5.\dfrac{1}{3}\left(1-\dfrac{1}{2020}\right)\)

Lấy máy tình CASIO bấm ra kết quả nha!

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