Áp dụng t/c dãy tỉ số bằng nhau:
a.
\(\dfrac{x}{9}=\dfrac{y}{7}=\dfrac{x+y}{9+7}=\dfrac{80}{16}=5\)
\(\Rightarrow\left\{{}\begin{matrix}x=9.5=45\\y=7.5=35\end{matrix}\right.\)
b.
\(\dfrac{x}{4}=\dfrac{y}{8}=\dfrac{z}{16}=\dfrac{x+y-z}{4+8-16}=\dfrac{32}{-4}=-8\)
\(\Rightarrow\left\{{}\begin{matrix}x=-8.4=-32\\y=-8.8=-64\\z=-8.16=-128\end{matrix}\right.\)
c.
\(\dfrac{x}{11}=\dfrac{y}{5}=\dfrac{z}{8}=\dfrac{x}{11}=\dfrac{2y}{10}=\dfrac{z}{8}=\dfrac{x-2y+z}{11-10+8}=\dfrac{108}{9}=12\)
\(\Rightarrow\left\{{}\begin{matrix}x=11.12=132\\y=5.12=60\\z=8.12=96\end{matrix}\right.\)
d.
\(\dfrac{x}{6}=\dfrac{y}{7}=\dfrac{z}{8}=\dfrac{3x}{18}=\dfrac{2y}{14}=\dfrac{5z}{40}=\dfrac{3x+2y-5z}{18+14-40}=\dfrac{80}{-8}=-10\)
\(\Rightarrow\left\{{}\begin{matrix}x=-10.6=-60\\y=-10.7=-70\\z=-10.8=-80\end{matrix}\right.\)