For since, as A is to B, so is C to D,
whatever part or parts A is of B,
the same part or parts is C of D also. [VII. Def. 20]
Therefore also the sum of A, C is the same part or the same parts of the sum of B, D that A is of B. [VII. 5, 6]
Therefore, as A is to B, so are A, C to B, D. [VII. Def. 20]