Cách 1:\(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a}{c}=\frac{b}{d}\Rightarrow\frac{a}{c}=\frac{a-b}{c-d}\Rightarrow\frac{a}{a-b}=\frac{c}{c-d}\)
Cách 2:\(\frac{a}{b}=\frac{c}{d}\Rightarrow ad=bc\Rightarrow ac-ad=ac-bc\Rightarrow a\left(c-d\right)=c\left(a-b\right)\Rightarrow\frac{a}{a-b}=\frac{c}{c-d}\)
Cách 3: Đặt\(\frac{a}{b}=\frac{c}{d}=m\Rightarrow a=mb;c=md\)
Ta có: \(\frac{a}{a-b}=\frac{mb}{mb-b}=\frac{mb}{b\left(m-1\right)}=\frac{m}{m-1}\)
\(\frac{c}{c-d}=\frac{md}{md-d}=\frac{md}{d\left(m-1\right)}=\frac{m}{m-1}\). Do đó \(\frac{a}{a-b}=\frac{c}{c-d}\)
Cách 4:\(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{b}{a}=\frac{d}{c}\Rightarrow1-\frac{b}{a}=1-\frac{d}{c}\Rightarrow\frac{a-b}{a}=\frac{c-d}{c}\Rightarrow\frac{a}{a-b}=\frac{c}{c-d}\)
Cách 5: \(\frac{a}{b}=\frac{c}{d}\Rightarrow ad=bc\).Do đó \(\frac{a}{a-b}=\frac{ad}{d\left(a-b\right)}=\frac{ad}{ad-bd}=\frac{ad}{bc-bd}=\frac{bc}{b\left(c-d\right)}=\frac{c}{c-d}\)
