1. tìm x biết a)1/5+x=-2/3 b)-5/8+x=4/9 c)1 3/4.x+1 1/2=-4/5 d)x(1/5+1/4)-(1/7+1/8)=0 2. Tính giá trị A=(-0,75-1/4) : (-5)+1/15-(-1/5):(-3) B=(3/25-1,

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1. Đáp án:

   1, a, $\dfrac{13}{15}$

   b, $\dfrac{77}{72}$

    c, $\dfrac{46}{35}$

   d, $-\dfrac{25}{27}$

   2, $A=\dfrac{1}{5}$

   $B=1$

   Giải thích các bước giải:

    1, 

   a, $\dfrac{1}{5}+x=-\dfrac{2}{3}\Leftrightarrow x=-\dfrac{2}{3}-\dfrac{1}{5}=\dfrac{13}{15}$

   b, $-\dfrac{5}{8}+x=\dfrac{4}{9}\Leftrightarrow x=\dfrac{4}{9}+\dfrac{5}{8}=\dfrac{77}{72}$

   c, $1\dfrac{3}{4}x+1\dfrac{1}{2}=-\dfrac{4}{5}\Leftrightarrow 1\dfrac{3}{4}x=-\dfrac{4}{5}-1\dfrac{1}{2}$

       $\Leftrightarrow 1\dfrac{3}{4}x=-\dfrac{23}{10}\Leftrightarrow x=-\dfrac{46}{35}$

   d, $x(\dfrac{1}{5}+\dfrac{1}{4})+(\dfrac{1}{7}+\dfrac{1}{8})=0\Leftrightarrow x\dfrac{9}{20}+\dfrac{15}{56}=0$
   $\Leftrightarrow x=-\dfrac{15}{56}:\dfrac{9}{20}=-\dfrac{25}{27}$

   2,

   $A=(-0,75-\dfrac{1}{4}):(-5)+\dfrac{1}{15}-(\dfrac{-1}{5}):(-3)$

   $=(-1):(-5)+\dfrac{1}{15}-\dfrac{1}{15}=\dfrac{1}{5}$

   $B=(\dfrac{3}{25}-1,12):\dfrac{3}{7}:[(3\dfrac{1}{2}-3\dfrac{2}{3}):\dfrac{1}{14}]$

   $=-1.\dfrac{7}{3}:(-\dfrac{1}{6}.14)=-\dfrac{7}{3}:(-\dfrac{7}{3})=1$

   Trả lời