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