\(5,\\ 5\cdot625=25\cdot125\\ \Leftrightarrow\dfrac{5}{25}=\dfrac{125}{625};\dfrac{5}{125}=\dfrac{25}{625};\dfrac{625}{25}=\dfrac{125}{5};\dfrac{625}{125}=\dfrac{25}{5}\)
\(6,\)
\(a,\) Áp dụng t/c dtsbn: \(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=\dfrac{-21}{7}=-3\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-3\cdot2=-6\\y=-3\cdot5=-15\end{matrix}\right.\)
\(b,7x=3y\Leftrightarrow\dfrac{x}{3}=\dfrac{y}{7}\)
Áp dụng t/c dtsbn: \(\dfrac{x}{3}=\dfrac{y}{7}=\dfrac{x-y}{3-7}=\dfrac{16}{-4}=-4\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-4\cdot3=-12\\y=-4\cdot7=-28\end{matrix}\right.\)
\(7,\\ a:b:c:d=2:3:4:5\Leftrightarrow\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}=\dfrac{d}{5}\)
Áp dụng t/c dtsbn: \(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}=\dfrac{d}{5}=\dfrac{a+b+c+d}{2+3+4+5}=\dfrac{-42}{14}=-3\)
\(\Leftrightarrow\left\{{}\begin{matrix}a=-3\cdot2=-6\\b=-3\cdot3=-9\\c=-3\cdot4=-12\\d=-3\cdot5=-15\end{matrix}\right.\)
