Answer :
[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.
If a/b=c/d (b≠0 and d≠0), then ad=bc.