a,b,c and dare real numbers such that

$\displaystyle \frac{a}{b} = \frac{c}{d} $ and b does not equal d,

show that

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

if i multiply by both sides by bd I get ad = bc so ad-bc = 0,

thats as far as i can go..any help appreciated.

thank you.