Answer :

konrad509
[tex]\dfrac{a}{b}=\dfrac{c}{d}\\ ad=bc [/tex]
naǫ
[tex]\frac{a}{b}=\frac{c}{d} \ \ \ |\times b \\ a=\frac{bc}{d} \ \ \ |\times d \\ ad=bc[/tex]

If a/b=c/d (b≠0 and d≠0), then ad=bc.