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

128 - (x +13) = 87

x + 13 = 128 - 87

x + 13 = 41

x= 41- 13

x = 28

120 x 13 + 120 x 87 - 120 x 10 = ?

= 120 x { 13 + 87 - 10 }

= 120 x  90

= 10800

120 x 13 + 120 x 87 - 120 x 10

= 120 x ( 13 + 87 - 10 )

= 120 x 90

= 10 800

120 x 13 + 120 x 87 - 120 x 10

= 120 x ( 13 + 87 - 10 )

= 120 x 90

= 10800

-1735,k

a)  -374

b)  -1800

          234 x 13 + 234 x 87 = 234 x (13 + 87)

                               = 234 x 100

                               = 23 400

tính bang cách thuận tiện: 735-48-35

234 x 13 + 234 x 87

=234 x (13+87)

=234 x 100

=23400

X=(1800+600):30):560:(315:35)

X=(2400:30):560:9

X=80:560:9

X=80x1/560x1/9

X=1/7x1/9

X=1/63

A=58x72+63x81-58x52-63x61

  = (58x72-58x52)+(63x81-63x61)

  = 58(72-52)+63(81-61)

  = 58x20+63x20

  = 1160+1260

  = 2420

B=87x96-27x13+13x27-87x86

  = (87x96-87x86)-(27x13-13x27)

  = 87(96-86)-27(13-13)

  = 87x10 - 27x0

  = 870-0

  = 870

C= 53x39+87x39-53x31-47-21

  = (53x39+87x39)-(53x31+47+21)

  = 39(53+87)-(53x31+68)

  = 39x140 - 53x31 -68

  = 5460 - 1643 - 68

  = 3749

=87.13-87.39-13.87-13.39 

=13.(87-87)-39.(87+13)

=13.0-39.100

=0-3900

=-3900

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

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 \(1230\)

Trả lời:

1, 13 . 87 + 13 . 12 + 13 = 13 ( 87 + 12 + 1 ) = 13 . 100 = 1300

2, ( x - 3 ) . 2x + ( x - 3 ). y 

= ( x - 3 ) ( 2x + y ) 

Thay x = 13; y = 4 vào bt trên, ta có:

( 13 - 3 ) ( 2.13 + 4 ) = 10 . 30 = 300

3, x( x + 4 ) - x (x -  6) 

= x ( x + 4 - x + 6 )

= 10x

Thay x = 123 vào bt trên, ta có:

10.123 = 1230