\(\frac{a}{b}=\frac{b}{d}\)
\(\Rightarrow\left(\frac{a}{b}\right)^2=\left(\frac{b}{d}\right)^2\)\(=\frac{a^2+b^2}{b^2+d^2}\)\(\)
Ta có: \(\frac{a^2+b^2}{b^2+d^2}\)\(=\left(\frac{a}{b}\right)^2\)
\(\Rightarrow\frac{a^2+b^2}{b^2+d^2}=\frac{a}{b}.\frac{a}{b}=\frac{a}{b}.\frac{b}{d}=\frac{a}{d}\)
\(\Rightarrow\frac{a^2+b^2}{b^2+d^2}=\frac{a}{d}\)