Bài 3: Cho ad = bc CMR : $\frac{a^{3}+b^{3}}{c^{3}+d^{3}}$ = $(\frac{a+b}{c+d})^{3}$

$ad = bc$

$\to \dfrac{a}{c} = \dfrac{b}{d}$

$\to \dfrac{a}{c} = \dfrac{b}{d} = \dfrac{a +b}{c + d}$

$\to \dfrac{a^3}{c^3} = \dfrac{b^3}{d^3} = \left(\dfrac{a+b}{c+d}\right)^3 = \dfrac{a^3 + b^3}{c^3 + d^3}$