Bài 1:

a) \(=\dfrac{8}{15}\left(\dfrac{7}{13}+\dfrac{6}{13}\right)=\dfrac{8}{15}.1=\dfrac{8}{15}\)

b) \(=\dfrac{3.3-7-2.4}{12}=-\dfrac{6}{12}=-\dfrac{1}{2}\)

Bài 2:

 \(\dfrac{x}{2,7}=-\dfrac{2}{3,6}\Rightarrow x=\dfrac{\left(-2\right).2,7}{3,6}\Rightarrow x=-\dfrac{3}{2}\)

Bài 3:

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=-\dfrac{21}{7}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=\left(-3\right).2=-6\\y=\left(-3\right).5=-10\end{matrix}\right.\)

\(\dfrac{8}{9}\) : ( 2 - 3 \(\times\) y) = \(\dfrac{5}{3}\) 

        2 - 3 \(\times\) y = \(\dfrac{8}{9}\) : \(\dfrac{5}{3}\)

        2 - 3 \(\times\) y = \(\dfrac{8}{15}\)

             3 \(\times\) y = 2 - \(\dfrac{8}{15}\)

             3 \(\times\) y = \(\dfrac{22}{15}\)

                   y  = \(\dfrac{22}{15}\) : 3 

                   y = \(\dfrac{22}{45}\)

( 2 x y + 2/15 ) x 3 = 4/5

( 2 x y + 2/15 )      = 4/5 : 3 

( 2 x y + 2/15 )      =   4/15

 2 x y                    = 4/15 - 2/15 

2 x y                     =     2/15

     y                      =     2/15 :2 

   y                          =    1/15

(2 x y + 2/15) x 3 = 4/5 

2 x y + 2/15) = 4/5 : 3 

2 x y + 2/15 = 4/15 

2 x y = 4/15 - 2/15 

2 x y = 2/15 

y = 2/15 : 2 

y = 1/15 

7/9 x (2 - 1/3 x y) = 14/15 

(2 - 1/3 x y) = 14/15 : 7/9 

(2 - 1/3 x y) = 6/5 

2 - y = 6/5 x 1/3 

2 - y = 2/5 

y = 2/5 + 2 

y = 12/5 

4/21 + 5 x y - 8/7 = 1/3 

4/21 + 5 x y = 1/3 + 8/7 

4/21 + 5 x y = 31/21 

5 x y = 31/21 - 4/21 

5 x y = 9/7 

y = 9/7 : 5 

y = 9/35 

7/12 x y - 3/12 x y = 5 

y x (7/12 - 3/12) = 5 

y x 1/3 = 5 

y = 5 : 1/3 

y = 15 

Giải:

a) \(\dfrac{-5}{8}=\dfrac{x}{16}\) 

\(\Rightarrow x=\dfrac{16.-5}{8}=-10\) 

\(\dfrac{3x}{9}=\dfrac{2}{6}\) 

\(\Rightarrow3x=\dfrac{2.9}{6}=3\) 

\(\Rightarrow x=1\)

b) \(\dfrac{x+3}{15}=\dfrac{1}{3}\)  

\(\Rightarrow x+3=\dfrac{1.15}{3}=5\) 

\(\Rightarrow x=2\)

\(\dfrac{6}{2x+1}=\dfrac{2}{7}\) 

\(\Rightarrow2x+1=\dfrac{6.7}{2}=21\) 

\(\Rightarrow x=10\)

c) \(\dfrac{4}{x-6}=\dfrac{y}{24}=\dfrac{-12}{18}\) 

\(\Rightarrow\dfrac{4}{x-6}=\dfrac{-12}{18}\) 

\(\Rightarrow x-6=\dfrac{18.4}{-12}=-6\) 

\(\Rightarrow x=0\) 

\(\Rightarrow\dfrac{y}{24}=\dfrac{-12}{18}\) 

\(\Rightarrow y=\dfrac{-12.24}{18}=-16\) 

 \(\dfrac{3-x}{-12}=\dfrac{16}{y+1}=\dfrac{192}{-72}\) 

\(\Rightarrow\dfrac{3-x}{-12}=\dfrac{192}{-72}\) 

\(\Rightarrow3-x=\dfrac{192.-12}{-72}=32\) 

\(\Rightarrow x=-29\) 

\(\Rightarrow\dfrac{16}{y+1}=\dfrac{192}{-72}\) 

\(\Rightarrow y+1=\dfrac{16.-72}{192}=-6\) 

d) \(\dfrac{-2}{3}< \dfrac{x}{5}< \dfrac{-1}{6}\) 

\(\Rightarrow\dfrac{-20}{30}< \dfrac{6x}{30}< \dfrac{-5}{30}\) 

\(\Rightarrow6x\in\left\{-18;-12;-6\right\}\) 

\(\Rightarrow x\in\left\{-3;-2;-1\right\}\) 

\(\dfrac{-1}{5}\le\dfrac{x}{8}\le\dfrac{1}{4}\) 

\(\Rightarrow\dfrac{-8}{40}\le\dfrac{5x}{40}\le\dfrac{10}{40}\) 

\(\Rightarrow5x\in\left\{-5;0;5;10\right\}\) 

\(\Rightarrow x\in\left\{-1;0;1;2\right\}\) 

e) \(\dfrac{x+46}{20}=x\dfrac{2}{5}\) 

\(\Rightarrow\dfrac{x+46}{20}=x+\dfrac{2}{5}\) 

\(\Rightarrow\dfrac{x+46}{20}=\dfrac{5x+2}{5}\) 

\(\Rightarrow5.\left(x+46\right)=20.\left(5x+2\right)\) 

\(\Rightarrow5x+230=100x+40\) 

\(\Rightarrow5x-100x=40-230\) 

\(\Rightarrow-95x=-190\) 

\(\Rightarrow x=-190:-95\) 

\(\Rightarrow x=2\) 

\(y\dfrac{5}{y}=\dfrac{86}{y}\) 

\(\Rightarrow y+\dfrac{5}{y}=\dfrac{86}{y}\) 

\(\Rightarrow\dfrac{y^2+5}{y}=\dfrac{86}{y}\) 

\(\Rightarrow y^2+5=86\) 

\(\Rightarrow y^2=86-5\) 

\(\Rightarrow y^2=81\) 

\(\Rightarrow\left[{}\begin{matrix}y=9\\y=-9\end{matrix}\right.\) 

Chúc bạn học tốt!

1.Tìm x,y thuộc Z biết

1,x+(-45)=(-62)+17

2,x+29=|-43|+(-43)

3,43+(9-21)=317-(x+317)

4,|x|+|-4|=7

5,|x|+|y|=0

6,(15-x)+(x-12)=7-(-5+x)

7,(2x-5)^2=9

8,(2x+6).(x-9)=0

9,(1-3x)^3=-8

10,3x+4y-xy=15

3.Tìm x+y biết

|x|=5

|x|=7

4.Tìm giá trị lớn nhất hoặc nhỏ nhất của các biểu thức sau (x,y thuộc Z)

A=|x-3|+1

B=3-|x+1|

C=|x-5|+|y+3|+7

1. y x 12 - y x 8 +y x 1 =15                     (y=yx1)

  (12-8+1) x y = 15

  5 x y = 15

y= 15:5

 y=3

2.       7 x y = 49-0

         7 x y = 49

             y = 49:7

            y= 7 

3.?????? sai đề àk

3) phải cho kết quả = bn mới tính đc chú

cái này có thật là toán lớp 3 hay k thế

Tìm y :

1 ) y x 12 - y x 8 + y = 15

    y x ( 12 - 8 + 1 ) = 15

    y x 5= 15

    y = 15 : 5

    y = 3

2 ) 49 - 7 x y = 0

    7 x y = 49 - 0

    7 x y = 49

          y = 49 : 7

          y = 7

3 ) y x 5 + y x 6

( câu này không có kết quả , làm sao mà giải ? )

a: \(\Leftrightarrow\dfrac{x}{-4}=\dfrac{21}{y}=\dfrac{z}{-80}=\dfrac{3}{4}\)

=>x=-3; y=28; z=-60

b: 5/12=x/-72

=>x=-72*5/12=-6*5=-30

c: =>x+3=-5

=>x=-8