Bài 2
a) Ta cần cminh
$\dfrac{a}{a-b} = \dfrac{c}{c-d}$
Ta có
$\dfrac{a}{a-b} = \dfrac{\frac{a}{b}}{\frac{a}{b} - 1} = \dfrac{\frac{c}{d}}{\frac{c}{d} - 1} = \dfrac{c}{c - d} = VP$.
b) Ta cần cminh
$\dfrac{a+b}{b} = \dfrac{c+d}{d}$
Ta có
$\dfrac{a+b}{b} = \dfrac{a}{b} + 1 = \dfrac{c}{d} + 1 = \dfrac{c+d}{d} = VP$.