Derivation of the Bayes' theorem can be made by the multiplication rule of probability and some algebra.
[Error: Macro 'mathequation' doesn't exist]
and
[Error: Macro 'mathequation' doesn't exist]
Subtracting these two equations leads:
[Error: Macro 'mathequation' doesn't exist]
and some algebraic transformation
[Error: Macro 'mathequation' doesn't exist]
yields the theorem
[Error: Macro 'mathequation' doesn't exist]