128 - ( x + 13 ) = 87 

           x + 13   = 128 - 87

           x + 13   = 41

           x          = 41 - 13

           x          = 28

2 . x - 13 = 29

2 . x        = 29 - 13

2 .x         = 16

    x         = 16 : 2

     x         = 8

128 - 3 . x = 92

        3 . x = 128 - 92

         3 . x = 36

              x = 36 : 3

              x = 12

546 : ( x + 17 ) = 13

           x + 17 = 546 : 13

           x + 17 = 42

           x         = 42 - 17

            x       = 25

128-(x+13)=87

=>x+13=128-87

=>x+13=41

=>x=41-13

=>x=28

128-(x+13)=87

x+13=128-87

x+13=41

x=41-13

x=28

B4

a)x=10

b)x=3

c)x=11

d)x=8

1.

a. 

11 + x : 5 = 13 

x : 5 = 13 - 11

x : 5 = 2

x      = 2 . 5

x      = 10

\(\dfrac{x+11}{89}+\dfrac{x+13}{87}=\dfrac{x+15}{85}+\dfrac{x+17}{83}\\\Leftrightarrow\left(\dfrac{x+11}{89}+1\right)+\left(\dfrac{x+13}{87}+1\right)=\left(\dfrac{x+15}{85}+1\right)+\left(\dfrac{x+17}{83}+1\right)\\ \Leftrightarrow\dfrac{x+100}{89}+\dfrac{x+100}{87}-\dfrac{x+100}{85}-\dfrac{x+100}{83}=0\\ \Leftrightarrow\left(x+100\right)\left(\dfrac{1}{89}+\dfrac{1}{87}-\dfrac{1}{85}-\dfrac{1}{83}\right)=0\\ \Leftrightarrow x+100=0\left(vì\dfrac{1}{89}+\dfrac{1}{87}-\dfrac{1}{85}-\dfrac{1}{83}\ne0\right)\\ \Leftrightarrow x=-100\)

1.\(13.87+13.12+13\)

\(=13\left(87+12+1\right)\)

\(=13.100=1300\)

2.Đề sai à ???

3.\(x\left(x+4\right)-x\left(x-6\right)\)

\(=x^2+4x-x^2+6x\)

\(=10x\)

\(=10.123=1230\)

1, \(13.87+13.12+13=13\left(87+12+1\right)=13.100=1300\)

2, bổ sung \(\left(x-3\right)2x+\left(x-3\right)y=\left(x-3\right)\left(2x+y\right)\)

Thay x = 13 ; y = 4 ta được : \(\left(13-3\right)\left(26+4\right)=10.30=300\)

3, \(x\left(x+4\right)-x\left(x-6\right)=x\left(x+4-x+6\right)=10x\)

Thay x = 123 ta được \(1230\)

A = ( -4/5 + 4/3 ) + (-5/4 + 14/5) - 7/3

  =   8/15 + 31/20 - 7/3

  =  25/12 - 7/3

  = -1/4

B = 8/3 x 2/5 x 3/8 x 10x 19/92

   = 16/15 x 15/4 x 19/92

   = 4x19/92

   = 19/23

C = - \(\dfrac{5}{7}\) x \(\dfrac{2}{11}\) + \(\dfrac{-5}{7}\) x \(\dfrac{9}{14}\) + \(\dfrac{1}{57}\)

   = - \(\dfrac{10}{77}\) - \(\dfrac{45}{98}\) + \(\dfrac{1}{57}\)

  = - \(\dfrac{635}{1078}\) + \(\dfrac{1}{57}\)

 = - \(\dfrac{36195}{61446}\) + \(\dfrac{1078}{61446}\)

 = - \(\dfrac{35117}{61446}\)

Ta thấy 17+17+.....+17 100 số có chữ số tận cùng là 0

13+13+.....+13 100 số 13 có chữ số tận cùng là 0

Vầy A-B.....

k nha

`1, -2/9 xx 15/17 + (-2/9) xx 2/17`

`= -2/9 xx (15/17 + 2/17)`

`= -2/9 xx 17/17`

`=-2/9xx1`

`=-2/9`

__

`-5/3 xx 6/5 + (-7/9) xx 3/10`

`= -30/15 + (-21/90)`

`= -2 + (-7/30)`

`=-60/30 +(-7/30)`

`=-67/30`

__

`15/20 xx 7/5 + (-9/7) xx (-6/4)`

`=3/4 xx7/5 + (-9/7) xx(-6/4)`

`= 21/20 + 54/28`

`= 21/20 + 27/14`

`=417/140`

__

`-25/13 xx 5/19 + (-25/13) xx 14/19`

`=-25/13 xx (5/19 +14/19)`

`=-25/13 xx 19/19`

`= -25/13 xx 1`

`=-25/13`

__

`-7/13 xx 13/5 + (-9/7) xx 5/3`

`=-7/5 +(-15/7)`

`=-124/35`