ta có : \(\frac{x}{y}=\frac{6}{9}\)
\(\Rightarrow\frac{x}{6}=\frac{y}{9}\)\(=\frac{x-y}{6-9}\)\(=\frac{30}{-3}\)= \(-10\)
\(\frac{x}{6}=-10\Rightarrow x=-60\)
\(\frac{y}{9}=-10\Rightarrow y=-90\)
b) \(\frac{x}{5}=\frac{y}{4}=\frac{z}{7}=\frac{x}{5}=\frac{2y}{8}=\frac{z}{7}=\frac{x+2y+z}{5+8+7}=\frac{40}{20}=2\)
\(\frac{x}{5}=2\Rightarrow x=10\)
\(\frac{y}{4}=2\Rightarrow y=8\)
\(\frac{z}{7}=2\Rightarrow z=14\)
C) ta có \(\frac{x}{3}=\frac{y}{4}\)
\(\Rightarrow\frac{x}{3}.\frac{1}{7}=\frac{y}{4}.\frac{1}{7}\)
\(\frac{x}{21}=\frac{y}{28}\)
ta lại có \(\frac{z}{5}=\frac{y}{7}\)
\(\Rightarrow\frac{z}{5}.\frac{1}{4}=\frac{y}{7}.\frac{1}{4}\)
\(\frac{z}{20}=\frac{y}{28}\)
\(\Rightarrow\frac{z}{20}=\frac{y}{28}=\frac{x}{21}\)\(=\frac{2x}{42}=\frac{3y}{84}=\frac{z}{20}=\frac{2x+3y-z}{42+84-20}=\frac{106}{106}=1\)
\(\frac{x}{21}=1\Rightarrow x=21\)
\(\frac{y}{28}=1\Rightarrow y=28\)
\(\frac{z}{20}=1\Rightarrow z=20\)
chúc bn hc tốt ^-^
