120-x=45-15

120-x=30

       x=120-30

       x= 90

120-x= 45-15

_____=30

x= 120-30

x=90

120-x+15=45

120-x      =45-15

120-x      =30

      x       =120-30

      x        =90

120 : x + 45 : x = 15

( 120 + 45 ) : x = 15

165 : x = 15

x = 165 : 15

x = 11

Vậy x = 11

Hok tốt

120 : x + 45 : x = 15

( 120 + 45 ) : x = 15

165 : x = 15

x = 11

120 : x + 45 : x = 15

=> ( 120 + 45 ) : x = 15

=> 165 : x = 15

=> x = 165 : 15 = 11

x = 30, y = 63, z = 168;

\(\dfrac{x}{42}=\dfrac{45}{y}=\dfrac{120}{z}=\dfrac{15}{21}\\ \cdot\dfrac{x}{42}=\dfrac{15}{21}\Rightarrow x=\dfrac{42.15}{21}=30\\ \cdot\dfrac{45}{y}=\dfrac{15}{21}\Rightarrow y=\dfrac{45.21}{15}=63\\ \cdot\dfrac{120}{z}=\dfrac{15}{21}\Rightarrow z=\dfrac{120.21}{15}=168\)

Vậy \(x=30;y=63;z=168\)

Ta có: \(\dfrac{x}{42}=\dfrac{45}{y}=\dfrac{120}{z}=\dfrac{15}{21}\)

\(\Leftrightarrow\dfrac{x}{42}=\dfrac{45}{y}=\dfrac{120}{z}=\dfrac{5}{7}\)

\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{x}{42}=\dfrac{5}{7}\\\dfrac{45}{y}=\dfrac{5}{7}\\\dfrac{120}{z}=\dfrac{5}{7}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{42\cdot5}{7}=30\\y=\dfrac{45\cdot7}{5}=63\\z=\dfrac{120\cdot7}{5}=168\end{matrix}\right.\)

Vậy: (x,y,z)=(30;63;168)

mình chịu thôi!lêu...lêu...lêu

4 . 120 + [45 : 9 +15 . (x + 1) = 500

480 + [45 : 9 +15 . (x + 1) = 500

45 : 9 +15 . (x + 1) = 500-480

45 : 9 +15 . (x + 1) = 20

5 +15 . (x + 1) = 20

15 . (x + 1) = 20-5

15 . (x + 1) = 15

x + 1=15:15

x + 1=1

x=1-1

x=0

= 0

tick cho mink nha

=>480 + [ 5 + 15 . (x + 1) = 500

15 . ( x + 1 )=500 - 480 - 5 = 15

x + 1 =15 : 15 =1

x =1 - 1

x = 0

vay x =0

k nha

a) 2x + 15 = 45

2x = 45 - 15

2x = 30

x = 30 : 2

x = 15 (nhận)

Vậy x = 15

b) 120 - 2.(x + 3) = 22.52

120 - 2.(x + 3) = 1144

2.(x + 3) = 120 - 1144

2.(x + 3) = - 1024

x + 3 = -1024 : 2

x + 3 = -512

x = - 512 - 3

x = -515 (loại)

Vậy không tìm được x thỏa mãn x là số tự nhiên

c) 11 ⋮ (x - 2)

⇒ x - 2 ∈ Ư(11) = {-11; -1; 1; 11}

⇒ x ∈ {-9; 1; 3; 13}

Do x là số tự nhiên

⇒ x ∈ {1; 3; 13}

d) Do 12 ⋮ x và 18 ⋮ x nên x ∈ ƯC(12; 18)

12 = 2².3

18 = 2.3²

ƯCLN(12; 18) = 2.3 = 6

⇒ x ∈ ƯC(12; 18) = {1; ; 3; 6}