Cross Multiply

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.