\(\dfrac{x}{5}=\dfrac{y}{6};\dfrac{y}{8}=\dfrac{z}{7}\) va \(x+y-z=69\)
Ta co: \(\dfrac{x}{5}=\dfrac{y}{6}\Rightarrow\dfrac{x}{20}=\dfrac{y}{24}\) ; \(\dfrac{y}{8}=\dfrac{z}{7}\Rightarrow\dfrac{y}{24}=\dfrac{z}{21}\)
➤ \(\dfrac{x}{20}=\dfrac{y}{24}=\dfrac{z}{21}\) ➤ \(\dfrac{x+y-z}{20+24-21}\)
➤ \(\dfrac{69}{23}=3\) ➤ \(x=20.3=60\)
\(y=24.3=72\)
\(z=21.3=63\)
\(Vay\) \(x=60;y=72;z=63\)
\(2a=3b;5b=7c\) va \(3a+5c-7c=30\)
Ta co: \(2a=3b\Rightarrow\dfrac{a}{3}=\dfrac{b}{2}\Rightarrow\dfrac{a}{21}=\dfrac{b}{14}\)
\(5b=7c\Rightarrow\dfrac{b}{7}=\dfrac{c}{5}\Rightarrow\dfrac{b}{14}=\dfrac{c}{10}\)
⇒ \(\dfrac{a}{21}=\dfrac{b}{14}=\dfrac{c}{10}\) ⇒ \(\dfrac{3a}{63}=\dfrac{5c}{50}=\dfrac{7b}{98}\) ⇒ \(\dfrac{3a+5c-7b}{63+50-98}\)
⇒ \(\dfrac{30}{15}=2\) ➤ \(3a=63.2=126\) ➤ \(a=126:3=42\)
\(5c=50.2=100\) \(c=100:5=20\)
\(7b=98.2=196\) \(b=196:7=28\)
Vay \(a=42;c=20;b=28\)
\(x\div y\div z=3\div8\div5\) va \(3x+y-2z=14\)
Ta co: \(x\div y\div z=3\div8\div5\Rightarrow\dfrac{x}{3}=\dfrac{y}{8}=\dfrac{z}{5}\)
⇒ \(\dfrac{3x}{9}=\dfrac{y}{8}=\dfrac{2z}{10}\) ⇒ \(\dfrac{3x+y-2z}{9+8-10}\)
⇒ \(\dfrac{14}{7}=2\) ➤ \(3x=9.2=18\) ➤ \(x=18:3=6\)
\(y=8.2\) \(y=16\)
\(2z=10.2=20\) \(z=20:2=10\)
Vay \(x=6;y=16;z=10\)
Chuc ban hoc tot 
