Hello dlbsdLeast Common Multiple -- from Wolfram MathWorld, about half-way down, from (10) to (14).
Q: ab/[a,b] is an integer. Prove that this integer is a common divisor of a and b. Deduce that ab/[a,b] is less than or equal to (a,b), the greatest common divisor of a and b.
([a,b]=lowest common multiple of a and b)
I have no idea how to start this one :S