For, with the same construction,
since the rectangle A, C is equal to the square on B,
while the square on B is the rectangle B, D,
for B is equal to D,
therefore the rectangle A, C is equal to the rectangle B, D.
But, if the rectangle contained by the extremes be equal to that contained by the means,
the four straight lines are proportional. [VI. 16]
Therefore, as A is to B, so is D to C.
But B is equal to D;
therefore, as A is to B, so is B to C.