Lê Quốc Trần Anh Coordinator
01/08/2017 at 08:55-
Nguyễn Thị Lan Hương 01/08/2017 at 17:03We have :
\(\dfrac{1}{3}+\dfrac{1}{31}+\dfrac{1}{35}+\dfrac{1}{37}+\dfrac{1}{47}+\dfrac{1}{53}+\dfrac{1}{61}\)
\(=\dfrac{1}{3}+\left(\dfrac{1}{31}+\dfrac{1}{35}+\dfrac{1}{37}\right)+\left(\dfrac{1}{47}+\dfrac{1}{53}+\dfrac{1}{61}\right)\)
\(< \dfrac{1}{3}+\left(\dfrac{1}{30}+\dfrac{1}{30}+\dfrac{1}{30}\right)+\left(\dfrac{1}{45}+\dfrac{1}{45}+\dfrac{1}{45}\right)\)
\(=\dfrac{1}{3}+\dfrac{1}{10}+\dfrac{1}{15}=\dfrac{1}{2}\)
=> \(\dfrac{1}{3}+\dfrac{1}{31}+\dfrac{1}{35}+\dfrac{1}{37}+\dfrac{1}{47}+\dfrac{1}{53}+\dfrac{1}{61}< \dfrac{1}{2}\)
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