Question

Cho C=\(\dfrac{1}{2}\cdot\dfrac{3}{4}\cdot\dfrac{5}{6}\cdot\cdot\cdot\dfrac{199}{200}\) Chứng minh C²<\(\dfrac{1}{201}\)

Answer 1

Ta có:\(C=\dfrac{1}{2}.\dfrac{3}{4}.....\dfrac{199}{200}\)

\(\Rightarrow C< \dfrac{2}{3}.\dfrac{4}{5}.....\dfrac{200}{201}\)

\(\Rightarrow C^2< \dfrac{2}{3}.\dfrac{4}{5}.....\dfrac{200}{201}.\dfrac{1}{2}.\dfrac{3}{4}.....\dfrac{199}{200}\)

\(\Rightarrow C^2< \dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}.\dfrac{4}{5}.....\dfrac{199}{200}.\dfrac{200}{201}\)

\(\Rightarrow C^2< \dfrac{1}{201}\) (đpcm)

Answer 2

Ta có :

\(C=\dfrac{1}{2}.\dfrac{3}{4}.\dfrac{5}{6}...\dfrac{199}{200}< \dfrac{2}{3}.\dfrac{4}{5}.\dfrac{6}{7}...\dfrac{200}{201}\)

\(\Rightarrow C^2< \dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}.\dfrac{4}{5}...\dfrac{199}{200}.\dfrac{200}{201}\)

\(\Rightarrow C^2< \dfrac{1.2.3.4....199.200}{2.3.4.5....200.201}=\dfrac{1}{201}\)

\(\Rightarrow\left(đpcm\right)\)