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Cho A=1/2.3/4.5/6.7/8…79/84. Chứng minh ràng:A<1/9 NHANH GIÚP EM NHÉ+GIẢI THÍCH NỮA Ạ
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Giải thích các bước giải:
Ta có:
$A=\dfrac12.\dfrac34.\dfrac56.\dfrac78….\dfrac{79}{80}$
$\to A<\dfrac23.\dfrac45.\dfrac67….\dfrac{90}{81}$
$\to A^2=(\dfrac12.\dfrac34.\dfrac56.\dfrac78….\dfrac{79}{80}).(\dfrac12.\dfrac34.\dfrac56.\dfrac78….\dfrac{79}{80})$
$\to A^2<(\dfrac12.\dfrac34.\dfrac56.\dfrac78….\dfrac{79}{80}).(\dfrac23.\dfrac45.\dfrac67….\dfrac{80}{81})$
$\to A^2<\dfrac12.\dfrac23.\dfrac34.\dfrac45.\dfrac56.\dfrac67…\dfrac{80}{81}$
$\to A^2<\dfrac1{81}$
$\to A<\dfrac19$