Tính nhanh: B=2/3.5+3/5.8+11/8.19+13/19.32+25/32.57+30/57.85

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1. B=$\frac{2}{3.5}$ + $\frac{3}{5.8}$+ $\frac{11}{8.19}$ +$\frac{13}{19.32}$ + $\frac{25}{32.57}$+ $\frac{30}{57.87}$ 
   B=$\frac{1}{3}$ – $\frac{1}{5}$ +$\frac{1}{5}$ -$\frac{1}{8}$+ $\frac{1}{8}$ -$\frac{1}{19}$+ $\frac{1}{19}$- $\frac{1}{32}$ +$\frac{1}{32}$ -$\frac{1}{57}$ +$\frac{1}{57}$ -$\frac{1}{87}$
   B =($\frac{1}{3}$ -$\frac{1}{87}$)+( $\frac{1}{5}$ -$\frac{1}{5}$ )+( $\frac{1}{8}$ -$\frac{1}{8}$ )+($\frac{1}{19}$ -$\frac{1}{19}$)+( $\frac{1}{32}$ -$\frac{1}{32}$ )+($\frac{1}{57}$ -$\frac{1}{57}$ )
   B=$\frac{1}{3}$ -$\frac{1}{87}$ 
   B=$\frac{28}{37}$ 
   Vậy B =$\frac{28}{37}$ 

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2. B=$\frac{2}{3.5}$ +$\frac{3}{5.8}$+$\frac{11}{8.19}$ +$\frac{13}{19.32}$+$\frac{25}{32.57}$+$\frac{30}{57.87}$    

      =$\frac{5-3}{3.5}$ +$\frac{8-5}{5.8}$+$\frac{19-8}{8.19}$ +$\frac{32-19}{19.32}$+$\frac{57-32}{32.57}$+$\frac{87-57}{57.87}$  

      =$\frac{1}{3}$-$\frac{1}{5}$+$\frac{1}{5}$ -$\frac{1}{8}$ +$\frac{1}{8}$-$\frac{1}{19}$ +$\frac{1}{19}$-$\frac{1}{32}$ +$\frac{1}{32}$-$\frac{1}{57}$+$\frac{1}{57}$ -$\frac{1}{87}$

      =$\frac{1}{3}$-$\frac{1}{87}$ =$\frac{28}{87}$  

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