1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90

0 bình luận về “1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90”

1. `1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90`

   `=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10`

   `=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10`

   `=1-1/10`

   `=9/10`

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2. Ta có $\frac{1}{2}$+ $\frac{1}{6}$+….+ $\frac{1}{90}$ 

   =$\frac{1}{1×2}$+ $\frac{1}{2×3}$+…+ $\frac{1}{9×10}$ 

   ADCT:$\frac{k}{n×(n+k)}$ =$\frac{1}{n}$- $\frac{1}{n+k}$ 

   ⇒1-$\frac{1}{2}$+ $\frac{1}{2}$+…..+ $\frac{1}{9}$- $\frac{1}{10}$ 

   ⇒1+($\frac{-1}{2}$+ $\frac{1}{2}$)+….+$\frac{-1}{10}$ 

   =1-$\frac{1}{10}$ 

   =$\frac{9}{10}$ 

   $Chúc,bạn,học,tốt,điểm,A+$

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