good morning guys,i have such question,when i was reading GRE book,there was such kind of property related to rational number,in shortly if we are trying to determine if $a/b$ is more then $c/d$ or vice verse,there was explained very short method,namely
$c/d>a/b$
if
$a*d>c*b$
but why it is so?i have tried following thing,let us least common multiple,which would be $d*b$; and on numerator place we have multiplies $b*c$ and $a*d$,it is clear for positive numbers,but for negative number are same?i means method for comparing rational numbers