For let AC, AD, FH, FK be joined.
Since then ABCM, ACDM are two pyramids which have triangular bases and equal height, they are to one another as the bases; [XII. 5]
therefore, as the base ABC is to the base ACD, so is the pyramid ABCM to the pyramid ACDM.
And, componendo, as the base ABCD is to the base ACD, so is the pyramid ABCDM to the pyramid ACDM. [V. 18]
But also, as the base ACD is to the base ADE, so is the pyramid ACDM to the pyramid ADEM. [XII. 5]
Therefore, ex aequali, as the base ABCD is to the base ADE, so is the pyramid ABCDM to the pyramid ADEM. [V. 22]
And again componendo, as the base ABCDE is to the base ADE, so is the pyramid ABCDEM to the pyramid ADEM. [V. 18]
Similarly also it can be proved that, as the base FGHKL is to the base FGH, so is the pyramid FGHKLN to the pyramid FGHN.
And, since ADEM, FGHN are two pyramids which have triangular bases and equal height,
therefore, as the base ADE is to the base FGH, so is the pyramid ADEM to the pyramid FGHN. [XII. 5]
But, as the base ADE is to the base ABCDE, so was the pyramid ADEM to the pyramid ABCDEM.
Therefore also, ex aequali, as the base ABCDE is to the base FGH, so is the pyramid ABCDEM to the pyramid FGHN. [V. 22]
But further, as the base FGH is to the base FGHKL, so also was the pyramid FGHN to the pyramid FGHKLN.
Therefore also, ex aequali, as the base ABCDE is to the base FGHKL, so is the pyramid ABCDEM to the pyramid FGHKLN. [V. 22]
Q. E. D.
Complete text
Book I