\(\left(12-34\right)-\left(34-8\right)=12-34-34+8\) 

                                                   \(=\left(-34-34\right)+\left(12+8\right)\)

                                                   \(=-68+20=-48\)

a) \(...=-\dfrac{12}{46}-\dfrac{15}{21}+\dfrac{3}{7}-\dfrac{7}{23}\)

\(=-\dfrac{12}{46}-\dfrac{7}{23}-\dfrac{15}{21}+\dfrac{3}{7}\)

\(=-\dfrac{6}{23}-\dfrac{7}{23}-\dfrac{5}{7}+\dfrac{3}{7}\)

\(=-\dfrac{13}{23}-\dfrac{2}{7}\)

\(=-\dfrac{13.7}{23.7}-\dfrac{2.23}{23.7}\)

\(=-\dfrac{91}{161}-\dfrac{46}{161}=-\dfrac{137}{161}\)

b) \(...=-21+\dfrac{7}{9}-\dfrac{3}{17}+\dfrac{25}{9}\)

\(=-21-\dfrac{3}{17}+\dfrac{7}{9}+\dfrac{25}{9}\)

\(=-21-\dfrac{3}{17}+\dfrac{32}{9}\)

\(=-\dfrac{3213}{153}-\dfrac{27}{153}+\dfrac{544}{153}=-\dfrac{2696}{153}\)

C,2x15x12+3x51x8+6x34

=2x3x5x12+3x51x2x4+6x34

=6x60+6x51x4+6x34

=6x60+6x204+6x34

=6x(60+204+34)

=6x298

=1788

đáp án là:

1788

chúc bạn học tốt !

\(2\times15\times12+3\times51\times8+6\times34\)

\(=2\times3\times5\times12+3\times51\times2\times4+6\times34\)

\(=\left(2\times3\right)\times5\times12+\left(3\times2\right)\times51\times4+6\times34\)

\(=6\times60+6\times204+6\times34\)

\(=6\times\left(60+204+34\right)\)

\(=6\times298\)

\(=1788\)

_Chúc bạn học tốt_

=23 bạn nhé

k mình nhé

= 23 nha bạn 

k cho mik nha

a: Số số hạng là:

(40-2):2+1=20(số)

Tổng là:

\(\dfrac{42\cdot20}{2}=42\cdot10=420\)

a: Số số hạng là:

(40-2):2+1=20(số)

Tổng là:

42x10=420

1) x - 18 : 3 = 12

x - 6 = 12

x = 12 + 6 

x = 18

2) ( x : 3 ) - 18 = 12

x : 3 = 12 + 18

x : 3 = 30

x = 30 x 3

'x = 90

3) ( x - 3 ) : 4 = 7

x - 3 = 7 x 4

x - 3 = 28

x = 28 + 3 

x = 31

4 ) ( x : 15 ) - 3 = 25 x 4

( x : 15 ) - 3 = 100

x : 15 = 100 + 3

x : 15 = 103

x = 103 x 15

x = 1545

5) ( x - 25 ) : 9 = 0

x - 25 = 0 x 9

x - 25 = 0

x = 0 + 25

x = 25

6) 34 : ( x - 8 ) = 34

x - 8 = 34 : 34

x - 8 = 1

x = 1 + 8

x = 9

7) ( x + 4 ) x 16 = 80

x + 4 = 80 : 16

x + 4 = 5

x = 5 - 4

x = 1

\(1.x-18:3=12\)

\(x-18=12.3\)

\(x-18=36\)

\(x=36+18\)

\(x=54\)

x+3666=102364

b: =20x(-2)+12x5

=-40+60

=20

`@` `\text {Ans}`

`\downarrow`

`1)`

`5/7*37 13/23 - 51 13/23*5/7`

`= 5/7* (37 13/23 - 51 13/23)`

`= 5/7* (-14)`

`= -10`

`2)`

`-2/3 +1/3+0,5+2 1/2`

`= -2/3 + 1/3 + 1/2 + 5/2`

`= (-2/3+1/3) + (1/2+5/2)`

`= -1/3 + 3`

`=8/3`

`3)`

`-0,5+2/3+1/2`

`= -1/2 + 2/3 + 1/2`

`= (-1/2 + 1/2) + 2/3`

`= 2/3`

`4)`

`(8+2 1/3-3/5) -(5+0,4)-(3 1/2 -2)`

`= 8+ 7/3 - 3/5 - 5 - 0,4 - 7/2 + 2`

`= (8+2-5) + (-3/5 - 2/5) + (7/3 - 7/2)`

`= 5 - 1 - 7/6`

`= 4 - 7/6 = 17/6`

`5)`

`(2/9-7/12):3/4+(16/9-5/12):3/4`

`= (2/9 - 7/12) \times 4/3 + (16/9 - 5/12) \times 4/3`

`= 4/3 *(2/9 - 7/12 + 16/9 - 5/12)`

`= 4/3 * [(2/9 + 16/9) + (-7/12 - 5/12)]`

`= 4/3 * ( 2 - 1)`

`= 4/3 * 1 = 4/3`

`6)`

`-(2021.0,7+19,75) +0,7- (8-19,75)`

`= -2021*0,7 -19,75 + 0,7 - 8 + 19,75`

`= 0,7*(-2021 + 1) - 8`

`= -1414-8`

`= -1422`

`7)`

`15/34+7/21+19/34-20/15`

`= (15/34 + 19/34) + 7/21 - 20/15`

`= 1 + 7/21 - 20/15`

`= 4/3 - 20/15 =0`

`8)`

`2 5/6+1/6:(-5/8)`

`= 17/6 + (-4/15)`

`= 77/30`

`9)`

`(-2)^2 +2/9. (4/5-2/3)`

`= 4 + 2/9*2/15`

`= 4+4/135`

`= 544/135`

`10)`

`(-1/5+3/7):5/4+(-4/5+4/7):5/4`

`= (-1/5+3/7) * 4/5 + (-4/5+4/7) * 4/5`

`= 4/5*(-1/5 +3/7-4/5+4/7)`

`= 4/5*[(-1/5-4/5)+(3/7+4/7)]`

`= 4/5* (-1+1)`

`= 4/5*0=0`

`11)`

`2022,2021 . 1954,1945+ 2022,2021 . (-1954,1945)`

`= 2022,2021 * [1954,1945 + (-1954,1945)]`

`= 2022,2021*0 `

`= 0`

`12)`

`-5,2 .72 +69,1 +5,2 . (-28)+(-1,1)`

`= -5,2*72 + 69,1 - 5,2*28 - 1,1`

`= -5,2*(72+28) + (69,1 - 1,1)`

`= -5,2*100 + 68`

`= -520 + 68`

`= -452`

`13)`

`(7 -1/2-3/4) : (5-1/4-5/8)`

`= 23/4 \div 33/8`

`=46/33`

`14)`

`(8+ 2 1/3 -3/5) -(5+0,4) -( 3 1/3 - 2)`

`= 8+ 2 1/3 - 3/5 - 5 - 0,4 - 3 1/3 + 2`

`= (8+2-5) + (2 1/3 - 3 1/3) - (0,6 + 0,4) `

`= 5 - 1 - 1`

`= 3`

help lười tính quá

2: \(=\dfrac{-1}{3}+3=\dfrac{8}{3}\)

3: =1/2-1/2+2/3=2/3

5: \(=\dfrac{4}{3}\left(\dfrac{2}{9}-\dfrac{7}{12}+\dfrac{16}{9}-\dfrac{5}{12}\right)=\dfrac{4}{3}\)

12: \(=5.2\left(-72-28\right)+68=-520+68=-452\)

13: \(\dfrac{23}{4}:\dfrac{33}{8}=\dfrac{46}{33}\)