If a = b and c = d, then a + c = b + d
The Partition Postulate:
AB + BC = AC
These are two different concepts. Let's try a proof:
As you can see, in step 2, we added equal quantities to each other. This is Addition Postulate.
But in step 3, we added a part plus a part equals a whole so this is Partition.
Let's try another example:
Here we used the Subtraction Postulate: subtracting equal quantities from equal quantities.
In step 3, we had the whole minus a part equals a part. This is still Partition Postulate.