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) \(x-15=-135\)

                \(x=-135+15\)

                \(x=-120\) 

Ta có: \(19\times26-17\times58+19\times74-17\times42\)

\(=19\times\left(26+74\right)-17\times\left(58+42\right)\)

\(=19\times100-17\times100=1900-1700=200\)

B

Chọn 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

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

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|

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

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

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

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

58x42+32x58+74x42

= 58x ( 42+ 32 ) + 74 x 42

=58 x 74 + 74 x 42

= 74 x ( 58+ 42)

=74 x 100

=7 400

58.(42+32)+74.42=58.74+74.42=74.(58+42)=74.100=7400