Tính nhanh 3,72 +2,05 + 6,28 + 7,95 = 1/10+2/20+3/30+4/40+5/50+6/60+7/70+8/80+9/90

a) 3,72 + 2,05 + 6,28 + 7,95

= (3,72 + 6,28) + (2,05 + 7,95)

= 10 + 10

= 20

b) $\frac{1}{10}$ + $\frac{2}{20}$ + $\frac{3}{30}$ + $\frac{4}{40}$ + $\frac{5}{50}$ + $\frac{6}{60}$ + $\frac{7}{70}$ + $\frac{8}{80}$ + $\frac{9}{90}$

= $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$ + $\frac{1}{10}$

= $\frac{9}{10}$ 

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