Ta có: \(\frac{a+3b}{a-3b}=\frac{c+3d}{c-3d}\)
\(\rightarrow\left(a+3b\right)\left(c-3d\right)=\left(a-3b\right)\left(c+3d\right)\)
\(\rightarrow ac+3bc-3ad-9bd=ac-3bc+3ad-9bd\)
\(\rightarrow3bc-3ad=3ad-3bc\)
\(\rightarrow6bc=6ad\)
\(\rightarrow bc=ad\rightarrow\frac{a}{c}=\frac{b}{d}\left(đpcm\right)\)
Chúc bn học tốt