$\begin{array}{l}có:\dfrac{a}{c}=\dfrac{b}{d}\\↔\dfrac{a}{c}.\dfrac{b}{d}=(\dfrac{a}{c})^2=(\dfrac{b}{d})^2\\↔\dfrac{ac}{bc}=(\dfrac{a}{c})^2=(\dfrac{b}{d})^2\\↔\dfrac{ac}{bc}=\dfrac{a^2+b^2}{c^2+d^2}\\\underline{\text{CHÚC BẠN HỌC TỐT}}\\\end{array}$
