Question

Bài 17 Tìm x biết :

a) \(\dfrac{x}{5}=\dfrac{2}{5}\)

b) \(\dfrac{3}{8}=\dfrac{6}{x}\)

c) \(\dfrac{1}{9}=\dfrac{x}{27}\)

d) \(\dfrac{4}{x}=\dfrac{8}{6}\)

e) \(\dfrac{3}{x-5}=\dfrac{-4}{x+2}\)

g) \(\dfrac{x}{-2}=\dfrac{-8}{x}\)

Answer 1

bài này áp dụng quy tắc nhân chéo nha :vv

a) \(\dfrac{x}{5}=\dfrac{2}{5}\Leftrightarrow5x=2.5=10\Leftrightarrow x=\dfrac{10}{5}=2\)

b) \(\dfrac{3}{8}=\dfrac{6}{x}\Leftrightarrow3x=6.8=48\Leftrightarrow x=\dfrac{48}{3}=16\)

c)\(\dfrac{1}{9}=\dfrac{x}{27}\Leftrightarrow9x=27\Leftrightarrow x=\dfrac{27}{9}=3\)

d)\(\dfrac{4}{x}=\dfrac{8}{6}\Leftrightarrow8x=4.6=24\Leftrightarrow x=\dfrac{24}{8}=3\)

e) \(\dfrac{3}{x-5}=\dfrac{-4}{x+2}\Leftrightarrow3\left(x+2\right)=-4\left(x-5\right)\\ \Leftrightarrow3x+4x=-6+20\\ \Leftrightarrow7x=14\Leftrightarrow x=\dfrac{14}{7}=2\)

g) \(\dfrac{x}{-2}=\dfrac{-8}{x}\Leftrightarrow x^2=\left(-2\right)\left(-8\right)=16\\ \Rightarrow x=\pm4\)

Answer 2

a)x=2

b)x=16

c)x=3

d)x=3

e)x=-2

g)x=4