Bài 1: tính a) -1+1/3 – 1/9 – 1/81 b) 4/5 – (- 2/7) – 7/10 c) 3/4 – [ 3/4-( 2/3 + 5/6)] bài 2: tính bằng 2 cách a) A=(7-3/4 +1/3)- (6+ 5/4 – 4/3)

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1. Giải thích các bước giải:

   1/ a/ $-1+\dfrac{1}{3}-\dfrac{1}{9}-\dfrac{1}{81}$

   $=\dfrac{-81+27-9-1}{81}$

   $=\dfrac{-64}{81}$

   b/ $\dfrac{4}{5}-(-\dfrac{2}{7})-\dfrac{7}{10}$

   $=\dfrac{4}{5}+\dfrac{2}{7}-\dfrac{7}{10}$

   $=\dfrac{56+20-49}{70}$

   $=\dfrac{27}{70}$

   c/ $\dfrac{3}{4}-[\dfrac{3}{4}-(\dfrac{2}{3}+\dfrac{5}{6})]$

   $=\dfrac{3}{4}-[\dfrac{3}{4}-\dfrac{4+5}{6}]$

   $=\dfrac{3}{4}-(\dfrac{3}{4}-\dfrac{3}{2})$

   $=\dfrac{3}{4}-\dfrac{3}{4}+\dfrac{3}{2}$

   $=\dfrac{3}{2}$

   2/ a/ C1: $A=(7-\dfrac{3}{4}+\dfrac{1}{3})-(6+\dfrac{5}{4}-\dfrac{4}{3})-(5-\dfrac{7}{4}+\dfrac{5}{3})$

   $=\dfrac{84-9+4}{12}-\dfrac{72+15-16}{12}-\dfrac{60-21+20}{12}$

   $=\dfrac{79}{12}-\dfrac{71}{12}-\dfrac{59}{12}$

   $=\dfrac{-51}{12}$

   $=-\dfrac{17}{4}$

   C2: $A=(7-\dfrac{3}{4}+\dfrac{1}{3})-(6+\dfrac{5}{4}-\dfrac{4}{3})-(5-\dfrac{7}{4}+\dfrac{5}{3})$

   $=7-\dfrac{3}{4}+\dfrac{1}{3}-6-\dfrac{5}{4}+\dfrac{4}{3}-5+\dfrac{7}{4}-\dfrac{5}{3}$

   $=(7-6-5)+(\dfrac{7}{4}-\dfrac{3}{4}-\dfrac{5}{4})+(\dfrac{1}{3}+\dfrac{4}{3}-\dfrac{5}{3})$

   $=-4-\dfrac{1}{4}+0$

   $=-\dfrac{17}{4}$

   b/ C1: $B=7+(\dfrac{7}{12}-\dfrac{1}{2}+3)-(\dfrac{1}{12}+5)$

   $=7+\dfrac{7-6+36}{12}-\dfrac{1+60}{12}$

   $=7+\dfrac{37}{12}-\dfrac{61}{12}$

   $=\dfrac{84+37-61}{12}$

   $=\dfrac{60}{12}$

   $=5$

   C2: $B=7+(\dfrac{7}{12}-\dfrac{1}{2}+3)-(\dfrac{1}{12}+5)$

   $=7+\dfrac{7}{12}-\dfrac{1}{2}+3-\dfrac{1}{12}-5$

   $=(7+3-5)+(\dfrac{7}{12}-\dfrac{1}{12})-\dfrac{1}{2}$

   $=5+\dfrac{6}{12}-\dfrac{1}{2}$

   $=5$

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2. Đáp án:

    

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