Let a rational straight line B be set out, and let the square on C be equal to the rectangle B, A;
therefore C is irrational; [X. Def. 4]
for that which is contained by an irrational and a rational straight line is irrational. [deduction from X. 20]
And it is not the same with any of the preceding;
for the square on none of the preceding, if applied to a rational straight line produces as breadth a medial straight line.
Again, let the square on D be equal to the rectangle B, C;
therefore the square on D is irrational. [deduction from X. 20]
Therefore D is irrational; [X. Def. 4]
and it is not the same with any of the preceding,
for the square on none of the preceding, if applied to a rational straight line, produces C as breadth.
Similarly, if this arrangement proceeds ad infinitum, it is manifest that from the medial straight line there arise irrational straight lines infinite in number, and none is the same with any of the preceding.
Q. E. D.
Complete text
Book I