Thao đề bài thì nó viết như bth nha :^
\(\dfrac{a}{c}=\dfrac{c}{b}=k\)
\(\Leftrightarrow\left\{{}\begin{matrix}a=ck\\c=bk\end{matrix}\right.\)
\(\dfrac{a^2+c^2}{b^2+c^2}=\dfrac{c^2k^2+b^2k^2}{b^2+c^2}=k^2\)
\(\dfrac{a}{b}=\dfrac{bk^2}{b}=k^2\)
Do đó: \(\dfrac{a^2+c^2}{b^2+c^2}=\dfrac{a}{b}\)