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Arată că( 1 pe 2*3)+ (1 pe 3*4)+(1 pe 4*5) .........+1 pe 2015*2016 <1 pe 2

Arată Că 1 Pe 23 1 Pe 341 Pe 45 1 Pe 20152016 Lt1 Pe 2 class=

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[tex]\displaystyle\\ \frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{2015\times2016}\\\\\text{Folosim formula }~\frac{1}{n\times(n+1)}=\frac{1}{n}-\frac{1}{n+1}\\\\\\ \frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{2015\times2016}=\\\\=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{2015}-\frac{1}{2016}=[/tex]


[tex]\displaystyle\\ \text{Reducem termenii asemenea.}\\\\=\frac{1}{2}-\frac{1}{2016}=\frac{1008}{2016}-\frac{1}{2016}=\frac{1008-1}{2016}= \frac{1007}{2016}\\\\\\\frac{1007}{2016}<\frac{1008}{2016}=\frac{1}{2}\\\\\implies~~\frac{1007}{2016}<\frac{1}{2}\\\\\implies~~\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{2015\times2016}<\frac{1}{2}[/tex]