a) \(74.42+74.58-400=74.\left(42+58\right)-400\)

                                                   \(=74.100-400\)

                                                   \(=7400-400\)

                                                   \(=7000\)

b) \(75-\left(3.5^2-4.2^3\right)=75-\left(3.25-4.8\right)\)

                                              \(=75-\left(75-32\right)\)

                                              \(=75-75+32\)

                                              \(=32\)

c) \(x-15=-135\)

                \(x=-135+15\)

                \(x=-120\) 

B

Chọn B

B

a, `12 + 126 + 42 + 74 + 158 + 88`

`= (12+88)+(126+74)+(42+158)`

`=100+200+200`

`=500`

b, `15xx23+15xx56+15xx21`

`=15xx(23+56+21)`

`=15xx100`

`=1500`

500

1500

bài này là tính nhanh nha

3/7 + 2/5=29/35

5/6 + 3=23/6

41/42 - 3/7= 23/42

4/5 - 1/2=3/10

3/7 + 2/5 = 15/35 + 14/35 = 29/35

5/6 + 3 = 5/6 + 18/6 = 11/3

41/42 - 3/7= 41/42 - 18/42 = 23/42

4/5 - 1/2 = 8/10 - 5/10= 3/10

3/7 + 2/5=15/35+14/35 = 29/35

5/6 + 3=5/6 + 3/1 = 5/6 + 18/6 = 23/6

41/42 - 3/7=41/42 - 18/42 = 23/42

4/5 - 1/2= 8/10 - 5/10 = 3/10

g) x - [ 42 + (-28)] = -8

\(\Rightarrow x-\left(42-28\right)=-8\)

\(\Rightarrow x-14=-8\)

\(\Rightarrow x=-8+14\)

\(\Rightarrow x=6\)

e) | x - 3| = |5| + | -7|

\(\Rightarrow\left|x-3\right|=5+7\)

\(\Rightarrow\left|x-3\right|=12\)

\(\Rightarrow\orbr{\begin{cases}x-3=12\\x-3=-12\end{cases}}\)

\(\Rightarrow\orbr{\begin{cases}x=15\\x=-9\end{cases}}\)

g) g) 4 - ( 7 - x) = x - ( 13 -4)

\(\Rightarrow4-7+x=x-9\)

\(\Rightarrow x-3=x-9\)

\(\Rightarrow x-x=-9+3\)

\(\Rightarrow vo-nghiem\)

a) x = 74    

b) x = 5 

c) x = 3      

d) x = 14