Toán 7 ( tìm x,y,z biết )

P

phamhuy20011801

a,
$y-2x=5$
$\leftrightarrow y=2x+5$
_Thế vào
$4x=5y$
$\leftrightarrow 4x=5(2x+5)$
$\leftrightarrow 4x=10x+25$
$\leftrightarrow x=\dfrac{-25}{6}$
...

C2: $4x=5y$
$\leftrightarrow \dfrac{x}{5}=\dfrac{2x}{10}=\dfrac{y}{4}=\dfrac{y-2x}{4-10}=\dfrac{-5}{6}$...
 
P

phamhuy20011801

b, $\dfrac{x}{2}=\dfrac{y}{3} \leftrightarrow \dfrac{x}{10}=\dfrac{y}{15}$
$\dfrac{y}{5}=\dfrac{z}{4} \leftrightarrow \dfrac{y}{15}=\dfrac{z}{12}$
$\rightarrow \dfrac{x}{10}=\dfrac{y}{15}=\dfrac{z}{12}=\dfrac{x-y+z}{10-15+12}=\dfrac{-49}{7}=-7$
$\rightarrow x=...; y=...; z=...$
 
W

windysnow

Ta có: $\frac{x}{2}$ = $\frac{y}{3}$ \Leftrightarrow $3x = 2y$ \Rightarrow $12x = 8y$ (1)

Ta có: $\frac{y}{5}$ = $\frac{z}{4}$ \Leftrightarrow $4y = 5z$ \Rightarrow $8y = 10z$ (2)

Từ (1) và (2) suy ra $12x = 8y = 10z$

Giải theo dãy tỉ số bằng nhau
 
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