Giải thích các bước giải:
a.Ta có :
$\dfrac ab=\dfrac cd\to\dfrac ac=\dfrac bd$
$\to\dfrac ac=\dfrac bd=\dfrac{a-b}{c-d}$
$\to (\dfrac{a-b}{c-d})^2=\dfrac ac.\dfrac bd=\dfrac{ab}{cd}$
b.Từ câu a
$\to \dfrac ac=\dfrac bd=\dfrac{a+b}{c+d}$
$\to \dfrac{a^3}{c^3}=\dfrac{b^3}{d^3}=(\dfrac{a+b}{c+d})^3=\dfrac{a^3-b^3}{c^3-d^3}$