bài1 tính hợp lí a) 1/6 +2/5 b) 3/5-7/4 c)5/13 +-5 /7+ -20/41 +8/13+-21/41 d) 1/2 +1/6 + 1/ 12 +1/20 +1/30

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1. Đáp án:

    

   Giải thích các bước giải:

    `a. 1/6 + 2/5 = 5/30 + 12/30 = 17/30`

   `b. 3/5 – 7/4 = 12/20 – 35/20= -23/20`

   `c. 5/13 +(-5 /7)+ (-20/41) +8/13+(-21/41)`

   `= (5/13 + 8/13) + [-20/41 + (-21/41)] + (-5/7)`

   `= 1 + (-1) + (-5/7)`

   `= -5/7`

   `d. 1/2 +1/6 + 1/ 12 +1/20 +1/30`

   `= (1/2 + 1/6+ 1/12) + 1/30+1/20`

   `=3/4 + 1/30+1/20`

   `= 45/60 + 2/60+3/60`

   `= 5/6`

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2. bài1) tính hợp lí

   `a) 1/6 +2/5 `

   `MC:30`

   =`1.5/6.5 +2.6/5.6`

   =`5/30+12/30`

   =`5+12/30`

   =`17/30`

   `b) 3/5-7/4`

   `MC:20`

   `=3.4/5.4-7.5/4.5`

   `=12/20-35/20`

   `=12-35/20`

   `=-23/20`

   $c)\frac{5}{13}+\frac{-5}{7}+\frac{-20}{41}+\frac{8}{13}+\frac{-21}{41}$

   $=(\frac{5}{13}+\frac{8}{13})+(\frac{-20}{41}+\frac{-21}{41})+\frac{-5}{7}$

   $=\frac{5+8}{13}+ \frac{-20-21}{13}+\frac{-5}{7}$

   $=1-1+\frac{-5}{7}$

   $=\frac{-5}{7}$

   `d) 1/2 +1/6 + 1/ 12 +1/20 +1/30`

   `=(1/2+1/12)+(1/20+1/30)+1/6`

   `=7/12+1/12+1/6`

   `=8/12+1/6`

   `=8/12+2/12`

   `=10/12`

   `=5/6`

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