`star` `color{red}{Answer}` `star`

Ta nhân chéo điều đề bài cho : 

 $\frac{a+b}{a-b}$ = $\frac{c+d}{c-d}$ 

`=>` `(a+b)(c-d)=(a-b)(c+d)`

`=>` `ac-ad+bc-bd=ac+ad-bc-bd`

`=>` `-ad+bc=ad-bc`

`=>` `2bc=2ad`

         `=>` bc=ad`

`=>` $\frac{a}{b}$ = $\frac{c}{d}$ `( Đpcm)`

        `\color{#bb8aff}{@}\color{#ac9bfd}{tr}\color{#9eacfc}{uo}\color{#8fbefa}{ng}\color \color{#bb8aff}{h}\color{#ac9bfd}{h}\color{#9eacfc}{uy}\color{#8fbefa}{mi}\color \color{#bb8aff}{nh}\color{#ac9bfd}{nh}\color{#9eacfc}{5313}\color`