What is componendo and dividendo method? Plz explain.

Componendo and dividendo is a method of simplification in Mathematics.

According to it,

This proof is invalid for a=b and c=d because if  such that a = b and c = d

then by application of componendo and dividendo, we get -

 [as a=b and c=d]

Since division by zero is not defined. So, this proof is invalid for a = b and c = d