Chọn câu sai?
\(\dfrac{A}{B}.\dfrac{B}{A}=1.\)\(\dfrac{A}{B}.\dfrac{C}{D}=\dfrac{C}{D}.\dfrac{A}{B}.\)\(\dfrac{A}{B}.\left(\dfrac{C}{D}.\dfrac{E}{F}\right)=\dfrac{E}{F}.\left(\dfrac{C}{D}.\dfrac{A}{B}\right).\)\(\dfrac{A}{B}\left(\dfrac{C}{D}+\dfrac{E}{F}\right)=\dfrac{A}{B}.\dfrac{C}{D}+\dfrac{E}{F}.\)