các bạn giúp mình nha!

N

nhithithu

Ta có:[TEX]A<\frac{1}{2^2}+\frac{1}{2.3}+\frac{1}{3.4}+...+[/TEX][TEX][/TEX][TEX]\frac{1}{9.10}[/TEX]
[TEX]A<\frac{1}{2^2}+[\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}][/TEX]
[TEX]A<\frac{1}{2^2}+\frac{1}{2}-\frac{1}{10}[/TEX]
[TEX]A<\frac{13}{20}<\frac{15}{20}=\frac{3}{4}[/TEX]
Vậy:[TEX]A<\frac{3}{4}[/TEX]
"Tích cho cái đúng"
 
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