Hitherto when two figures have been said to be equal, it has been implied that they are identically equal, that is , equal in all respects.
In Section III. of Euclid’s first Book, we have to consider the equality in area of parallelograms and triangles which are not necessarily equal in all respects.
[The ultimate test of equality, as we have already seen, is afforded by Axiom 8, which asserts that magnitudes which may be made to coincide with one another are equal. Now figures which are not identically equal, cannot be made to coincide without first undergoing some change of from: hence the method of direct superposition is unsuited to the purposes of the present section.
We shall see however from Euclid’s proof of Proposition 35, that two figures which are not identically equal, may nevertheless be so related to a third figure, that it is possible to infer the equality of their areas.]