Giải thích các bước giải:
a.Ta có :
$\dfrac ab=\dfrac cd$
$\to\dfrac{a^2}{b^2}=\dfrac{c^2}{d^2}=\dfrac{ac}{bd}$
$\to\dfrac{a^2}{b^2}=\dfrac{5c^2}{5d^2}=\dfrac{ac}{bd}=\dfrac{a^2+5c^2}{b^2+5d^2}$
$\to đpcm$
b.Ta có :
$\dfrac ab=\dfrac cd$
$\to\dfrac{a^2}{b^2}=\dfrac{c^2}{d^2}=\dfrac{ac}{bd}$
$\to\dfrac{a^2}{b^2}=\dfrac{2c^2}{2d^2}=\dfrac{ac}{bd}=\dfrac{a^2-2c^2}{b^2-2d^2}$