\(\frac{7x+5y}{3x-7y}=\frac{7z+5t}{3z-7t}\Rightarrow\left(7x+5y\right)\left(3z-7t\right)=\left(7z+5t\right)\left(3x-7y\right)\)
\(\Leftrightarrow21xz-49xt+15yz-35yt=21xz-49yz+15xt-35yt\)
\(\Leftrightarrow-49xt+15yz=-49yz+15xt\Leftrightarrow-64xt=-64yz\Leftrightarrow xt=yz\Leftrightarrow\frac{x}{y}=\frac{z}{t}\)
Vậy ta có đpcm