Thực hiện phép tính : A = 2/3.5 + 2/5.7 + 2/7.9 + …. + 2/61.63 B = 2/1.4 + 2/4.7 + 2/7.11 + …. +2/100.103

2 bình luận về “Thực hiện phép tính : A = 2/3.5 + 2/5.7 + 2/7.9 + …. + 2/61.63 B = 2/1.4 + 2/4.7 + 2/7.11 + …. +2/100.103”

1. A = $\frac{2}{3.5}$ + $\frac{2}{5.7}$ + $\frac{2}{7.9}$ + … + $\frac{2}{61.63}$ 

   A = $\frac{1}{3}$ – $\frac{1}{5}$ + $\frac{1}{5}$ – $\frac{1}{7}$ + $\frac{1}{7}$ – $\frac{1}{9}$ + … + $\frac{1}{61}$ – $\frac{1}{63}$ 

   A = $\frac{1}{3}$ – $\frac{1}{63}$ 

   A = $\frac{20}{63}$ 

   B = $\frac{2}{1.4}$ + $\frac{2}{4.7}$ + $\frac{2}{7.11}$ + … + $\frac{2}{100.103}$ 

   B = $\frac{2}{3}$ . ( $\frac{3}{1.4}$ + $\frac{3}{4.7}$ – $\frac{3}{7.11}$ + … + $\frac{3}{100.103}$

   B = $\frac{2}{3}$ . ( $\frac{1}{1}$ – $\frac{1}{4}$ + $\frac{1}{4}$ – $\frac{1}{7}$ + $\frac{1}{7}$ – $\frac{1}{11}$ + … + $\frac{1}{100}$ – $\frac{1}{103}$ )

   B = $\frac{2}{3}$ . ( $\frac{1}{1}$ – $\frac{1}{103}$ )

   B = $\frac{2}{3}$ . $\frac{102}{103}$ 

   B = $\frac{68}{103}$ .

   Trả lời
2. Giải đáp + Lời giải và giải thích chi tiết:

   A=2/3.5+2/5.7+2/7.9+…+2/61.63

   =1/3-1/5+1/5-1/7+1/7-1/9+…+1/61-1/63

   =1/3-1/63

   =21/63-1/63

   =(21-1)/63

   =20/63

   Vậy A=20/63

   __

   B=2/1.4+2/4.7+2/7.11+…+2/100.103

   =2 . 1/3 . ( 3/1.4 + 3/4.7 + 3/7.11 + … + 3/100.103)

   =2/3 . (1 – 1/4 + 1/4 – 1/7 +1/7 – 1/11 + … + 1/100 – 1/103)

   =2/3 . (1-1/103)

   =2/3 . (103/103-1/103)

   =2/3 . 102/103

   =(2.102)/(3.103)

   =68/103

   Vậy B=68/103

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   Trả lời