Let a∣c and b∣c such that greatest common divisor (gcd) gcd, Show that ab\mid c.
Answer
HINT \rm\quad\ \ a,b\ |\ c\ \Rightarrow\ ab\ |\ ac,bc\ \Rightarrow\ ab\ |\ (ac,bc) = (a,b)\:c = c\ via \rm\:(a,b)= 1\:.
NOTE \ This proof works in every domain where GCDs exist, since it doesn't use Bezout's identity. Instead it uses the GCD distributive law \rm\ (ac,bc) = (a,b)\:c\:,\ true in every GCD domain, viz.
LEMMA \rm\quad (a,b)\ =\ (ac,bc)/c\ \ if \rm\ (ac,bc)\ exists.
Proof \rm\quad\ d\ |\ a,b\ \iff\ dc\ |\ ac,bc\ \iff\ dc\ |\ (ac,bc)\ \iff\ d\ |\ (ac,bc)/c
The above proofs use the universal definitions of GCD, LCM, which often serve to simplify proofs, e.g. see this proof of the GCD * LCM law.
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