For, whatever part AE is of CF, the same part also let EB be of CG.
Now since, whatever part AE is of CF, the same part also is EB of CG,
therefore, whatever part AE is of CF, the same part also is AB of GF. [VII. 5]
But, whatever part AE is of CF, the same part also, by hypothesis, is AB of CD;
therefore, whatever part AB is of GF, the same part is it of CD also;
therefore GF is equal to CD.
Let CF be subtracted from each;
therefore the remainder GC is equal to the remainder FD.
Now since, whatever part AE is of CF, the same part also is EB of GC,
while GC is equal to FD,
therefore, whatever part AE is of CF, the same part also is EB of FD.
But, whatever part AE is of CF, the same part also is AB of CD;
therefore also the remainder EB is the same part of the remainder FD that the whole AB is of the whole CD.
Q. E. D.
Complete text
Book I