and, if
MN be subtracted from each,
LM is also in excess of NP; so that, if
GH is in excess of
KO,
LM is also in excess of
NP.
Similarly we can prove that, if
GH be equal to
KO,
LM will also be equal to
NP, and if less, less.
And
GH,
LM are equimultiples of
AE,
CF, while
KO,
NP are other, chance, equimultiples of
EB,
FD;
therefore, as AE is to EB, so is CF to FD.
Therefore etc. Q. E. D.
PROPOSITION 18.
If magnitudes be proportional
separando,
they will also be proportional
componendo.
Let
AE,
EB,
CF,
FD be magnitudes proportional
separando, so that, as
AE is to
EB, so is
CF to
FD; I say that they will also be proportional
componendo, that is, as
AB is to
BE, so is
CD to
FD.
For, if
CD be not to
DF as
AB to
BE, then, as
AB is to
BE, so will
CD be either to some magnitude less than
DF or to a greater.
First, let it be in that ratio to a less magnitude
DG.
Then, since, as
AB is to
BE, so is
CD to
DG, they are magnitudes proportional
componendo;
so that they will also be proportional separando. [V. 17]
Therefore, as
AE is to
EB, so is
CG to
GD.
But also, by hypothesis,
as AE is to EB, so is CF to FD.
Therefore also, as
CG is to
GD, so is
CF to
FD. [
V. 11]
But the first
CG is greater than the third
CF;
therefore the second GD is also greater than the fourth FD. [V. 14]
But it is also less: which is impossible.
Therefore, as
AB is to
BE, so is not
CD to a less magnitude than
FD.