If two fractions are equal, say
then it is always true that the products of the numbers given by
are also equal or ad = bc. This is the most common form of cross multiplication. That
implies ad = bc can be shown by multiplying both sides of
by the common denominator bd and canceling.
Cross multiplying is a common first step in solving proportions.
is equivalent to
Therefore, x = 9.
Adding fractions can also be done with cross multiplication.
Note that the cross product of two vectors does not involve cross multiplying.