1. \(\dfrac{x}{19}=\dfrac{y}{21};2x-y=34\)
Có: \(\dfrac{x}{19}=\dfrac{y}{21}\)
=> \(\dfrac{2x}{38}=\dfrac{y}{21}\)
Theo tính chất dãy tỉ số bằng nhau ta có:
\(\dfrac{2x}{38}=\dfrac{y}{21}=\dfrac{2x-y}{38-21}=\dfrac{34}{17}=2\)
=> \(\dfrac{x}{19}=2=>x=2.19=38\)
=> \(\dfrac{y}{21}=2=>y=2.21=42\)
Vậy x= 38 ; y= 42
2. \(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\);\(2x+3y-z=186\)
Có: \(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\)
=> \(\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}=\dfrac{2x+3y-z}{30+60-28}=\dfrac{186}{62}=3\)
=> \(\dfrac{x}{15}=3=>x=3.15=45\)
=>\(\dfrac{y}{20}=3=>y=3.20=60\)
=> \(\dfrac{z}{28}=3=>z=3.28=84\)
Vậy x=45;y=60;z=84
1) \(\dfrac{x}{19}=\dfrac{y}{21}\) và 2x -y =34
Từ \(\dfrac{x}{19}=\dfrac{y}{21}=>\dfrac{2x}{38}=\dfrac{y}{21}\)
Áp dụng tính chất của dãy tỉ số bằng nhau,ta có:
\(\dfrac{2x}{38}=\dfrac{y}{21}=\dfrac{2x-y}{38-21}=\dfrac{34}{17}=2\)
=>\(\dfrac{2x}{38}=2=>2x=2.38=>2x=76=>x=76:2=>x=38\)
=>\(\dfrac{y}{21}=2=>y=2.21=>y=42\)
Vậy x=38; y=42
2)\(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\)và 2x+3y-z=186
Từ \(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}=>\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}\)
Áp dụng t/c của dãy tỉ số bằng nhau,ta có:
\(\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}=\dfrac{2x+3y-z}{30+60-28}=\dfrac{186}{62}=3\)
=>\(\dfrac{2x}{30}=3=>2x=3.30=>2x=90=>x=90:2=>x=45\)
=>\(\dfrac{3y}{60}=3=>3y=3.60=>3y=180=>y=180:3=>y=60\)
=>\(\dfrac{z}{28}=3=>z=3.28=>z=84\)
Vậy x=45; y=60; z=84
3)\(\dfrac{x}{3}=\dfrac{y}{4}\) và\(\dfrac{y}{5}=\dfrac{z}{7}\)và 2x+3y-z=372
Từ\(\dfrac{x}{3}=\dfrac{y}{4}=>\dfrac{x}{15}=\dfrac{y}{20}\)
\(\dfrac{y}{5}=\dfrac{z}{7}=>\dfrac{y}{20}=\dfrac{z}{28}\)
=>\(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}=>\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}\)
Áp dụng t/c của dãy tỉ số bằng nhau,ta có:
\(\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{z}{28}=\dfrac{2x+3y-z}{30+60-28}=\dfrac{372}{62}=6\)
=>\(\dfrac{2x}{30}=6=>2x=6.30=>2x=180=>x=180:2=>x=90\)
=>\(\dfrac{3y}{60}=6=>3y=6.60=>3y=360=>y=360:3=>y=120\)
=>\(\dfrac{z}{28}=6=>z=6.28=>z=148\)
Vậy x=90; y=120; z=148