cho a= 5/1.2.3+8/2.3.4+11/3.4.5+..+59/19.20.21 cmr a<2

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1. Bài: A = $\frac{5}{1.2.3}$ + $\frac{8}{2.3.4}$ + $\frac{11}{3.4.5}$ + … + $\frac{59}{19.20.21}$ 
   Ta có:
   $\frac{5}{1.2.3}$ < 2
   $\frac{8}{2.3.4}$ < 2
   $\frac{11}{3.4.5}$ < 2
   …
   $\frac{59}{19.20.21}$ < 2

   ⇒ A < 2

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