Giải:

a) \(\dfrac{7}{x}=\dfrac{x}{28}\)

\(\Leftrightarrow x^2=196\)

\(\Leftrightarrow x=\pm\sqrt{196}=\pm14\)

Vậy ...

b) \(\dfrac{10+2}{17+x}=\dfrac{3}{4}\)

\(\Leftrightarrow40+8=51+3x\)

\(\Leftrightarrow3x=40+8-51=-3\)

\(\Leftrightarrow x=-\dfrac{3}{3}=-1\)

Vậy ...

c) \(\dfrac{40+x}{17+x}=\dfrac{6}{7}\)

\(\Leftrightarrow280+7x=102+6x\)

\(\Leftrightarrow7x-6x=102-280\)

\(\Leftrightarrow x=-178\)

Vậy ...

Tìm x biết

1.-1/4-x=-3/2

2.3/5.(x-1/2):3/5-1/3=-1/2

3.(x-1/2):3/5-1/3=-1/2

4.1/2-|x-1/2|=2/5

5.1/2-|x-1/2|=2/3

6.3/4+|x-1/2018|=1/4_-1/2

7.|x+2|=|1-|12+1/3|+1/2

1. -1/4 - x = -3/2

x = -1/4 - -3/2

x = -1/4 - -6/4

x = 5/4

2. 3/5 . (x - 1/2) : 3/5 - 1/3 = -1/2

3/5 . (x - 1/2) : 3/5 = -1/2 + 1/3

3/5 . (x - 1/2) : 3/5 = -3/6 + 2/6

3/5 . (x - 1/2) : 3/5 = -1/6

3/5 . (x - 1/2) = -1/6 . 3/5

3/5 . (x - 1/2) = -1/10

x - 1/2 = -1/10 : 3/5

x - 1/2 = -1/6

x = -1/6 + 1/2

x = -1/6 + 3/6

x = 2/6

x = 1/3

Mấy câu sau bn tự làm nha!