IX.16 Proposition IX.16
If two numbers be prime to one another, the second will not be to any other number as the first is to the second.
Proof
If a:b=b:c with gcd(a,b)=1, then b2=ac, forcing
a∣b2 which (Euclid's lemma) forces a∣b – contradicting
coprimality unless a=1.
lines 74–74 in main.tex