=2.x= -5.(-4)

= -5 . (-4) trên 2

=Vậy x= 10

-3/21+-2/7

thực hiện phép tính         

  -3/21+-2/7

\(a.32,5-3\cdot0,87=32,5-2,61=29,89\)

\(8,5\cdot\left(1\dfrac{1}{2}+\dfrac{4}{4}\right):5=8,5\cdot\left(\dfrac{3}{2}+\dfrac{4}{4}\right):5\\ =8,5\cdot\left(\dfrac{6}{4}+\dfrac{4}{4}\right):5\\ =8,5\cdot\dfrac{10}{4}:5\\ =\dfrac{85}{4}:5\\ =\dfrac{17}{4}\)

\(b.30,96-6,45+14,4:3=30,96-6,45+4,8\\ =29,31\)

\(\dfrac{2}{5}\cdot\left(\dfrac{4}{5}-\dfrac{1}{2}\right)=\dfrac{2}{5}\cdot\left(\dfrac{8}{10}-\dfrac{5}{10}\right)\\ =\dfrac{2}{5}\cdot\dfrac{3}{10}=\dfrac{3}{25}\)

bài 2

\(a.2,5\cdot12,5\cdot8\cdot0,4=\left(2,5\cdot0,4\right)\left(12,5\cdot8\right)\\ =1\cdot100=100\)

b,\(\dfrac{12}{15}\cdot\dfrac{5}{6}\cdot\dfrac{3}{20}\cdot\dfrac{32}{5}=\dfrac{12\cdot5\cdot3\cdot32}{15\cdot6\cdot20\cdot5}\\ =\dfrac{3\cdot4\cdot5\cdot3\cdot4\cdot8}{3\cdot5\cdot2\cdot3\cdot5\cdot4\cdot5}=\dfrac{16}{25}\)

Bài 1: 

a) \(32.5-3\cdot0.87=32.5-2.61=29.89\)

\(8.5\cdot\left(1\dfrac{1}{2}+\dfrac{4}{4}\right):5=8.5\cdot\dfrac{5}{2}:5=\dfrac{17}{2}\cdot\dfrac{5}{2}:5=\dfrac{85}{4}\cdot\dfrac{1}{5}=\dfrac{17}{4}\)

b) \(30.96-6.45+14.4:3=24.51+4.8=29.33\)

\(\dfrac{2}{5}\left(\dfrac{4}{5}-\dfrac{1}{2}\right)=\dfrac{2}{5}\left(\dfrac{8}{10}-\dfrac{5}{10}\right)=\dfrac{2}{5}\cdot\dfrac{3}{10}=\dfrac{6}{50}=\dfrac{3}{25}\)

em nên gõ công thức trực quan để đề bài rõ ràng nhé

bn viết tke bố tôi cx ko hiểu đc

12 phần 25 trừ cho 14 phần 5 bằng bao nhiêu

8==D LOL

1,

\(\frac{25}{12}+\left(\frac{-4}{12}\right)=\frac{7}{4}\)

\(\frac{-10}{8}+\frac{15}{4}=\frac{5}{2}\)

\(\frac{3}{8}+\frac{-14}{6}=\frac{-47}{24}\)

\(\frac{350}{150}+\left(\frac{-200}{360}\right)=\frac{16}{9}\)

\([\frac{5}{8}+\left(\frac{-3}{4}\right)]+\frac{15}{6}=\frac{-1}{8}+\frac{15}{6}=\frac{19}{8}\)

\(\frac{7}{3}+[\left(\frac{-5}{6}\right)+\left(\frac{-2}{3}\right)]=\frac{7}{3}+\left(\frac{-3}{2}\right)=\frac{5}{6}\)

4,

\(\frac{X+1}{5}=\frac{3}{7}\)

\(\Rightarrow\left(X-1\right).7=3.5\)

\(\Rightarrow7X-7=15\)

\(\Rightarrow7X=22\)

\(\Rightarrow X=\frac{22}{7}\)

\(\frac{X-3}{4}=\frac{1}{2}\)

\(\Rightarrow\left(X-3\right)2=1.4\)

\(\Rightarrow2X-6=4\)

\(\Rightarrow2X=10\)

\(\Rightarrow X=5\)

Tôi ko bt

\(\dfrac{2}{x+1}=\dfrac{4}{x-5}\\ ĐK:\left\{{}\begin{matrix}x\ne-1\\x\ne5\end{matrix}\right.\\ \Leftrightarrow2.\left(x-5\right)=4.\left(x+1\right)\\ \Leftrightarrow2x-10=4x+4\\ \Leftrightarrow2x-4x=4+10\\ \Leftrightarrow-2x=14\\ \Leftrightarrow x=-7\left(t/m\right)\)

\(\dfrac{2}{x+1}=\dfrac{4}{x-5}\left(x\ne-1;x\ne5\right)\)

suy ra: \(2\left(x-5\right)=4\left(x+1\right)\\ < =>2x-10=4x+4\\ < =>2x-4x=4+10\\< =>-2x=14\\ < =>x=-7\)

\(\dfrac{2}{x+1}=\dfrac{4}{x-5}\\ \Leftrightarrow2.\left(x-5\right)=4.\left(x+1\right)\\ \Leftrightarrow2x-10=4x+4\\ \Leftrightarrow2x-4x=10+4\\ \Leftrightarrow-2x=14\\ \Leftrightarrow x=14:\left(-2\right)\\ \Leftrightarrow x=-7\)