Đặt \(\frac{a}{c}=\frac{c}{b}=k\Rightarrow a=kc;c=kb\)(1)
Thay (1) vào ta có : \(\frac{a^2+c^2}{c^2+b^2}=\frac{\left(kc\right)^2+c^2}{\left(kb\right)^2+b^2}=\frac{k^2.c^2+c^2}{k^2.b^2+b^2}=\frac{c^2\left(k^2+1\right)}{b^2\left(k^2+1\right)}=\frac{c^2}{b^2}=\frac{\left(kb\right)^2}{b^2}=k^2\)
\(\RightarrowĐPCM\)