Question

[Nguyen Ngoc Linh]

tinh giá trị biểu thức sau ; A=7/4.(3333/1212+3333/2020+3333/3030+3333/4242)

Answer 1

[Lightning Farron]

\(A=\dfrac{7}{4}\left(\dfrac{3333}{1212}+\dfrac{3333}{2020}+\dfrac{3333}{3030}+\dfrac{3333}{4242}\right)\)

\(=\dfrac{7}{4}\left(\dfrac{33\cdot101}{12\cdot101}+\dfrac{33\cdot101}{20\cdot101}+\dfrac{33\cdot101}{30\cdot101}+\dfrac{33\cdot101}{42\cdot101}\right)\)

\(=\dfrac{7}{4}\left(\dfrac{33}{12}+\dfrac{33}{20}+\dfrac{33}{30}+\dfrac{33}{42}\right)\)

\(=\dfrac{7}{4}\cdot33\left(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}\right)\)

\(=\dfrac{231}{4}\left(\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}\right)\)

\(=\dfrac{231}{4}\left(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\right)\)

\(=\dfrac{231}{4}\left(\dfrac{1}{3}-\dfrac{1}{7}\right)=\dfrac{231}{4}\cdot\dfrac{4}{21}=11\)