Show that if a and b are positive integers, then the greatest common di-
visor of a and b divides their least common multiple. When are the least
common multiple and the greatest common divisor equal to each other?
As a is a multiple of gcd (a; b), we can write a = gcd (a; b) i. As lcm (a; b)
is a multiple of a, we can write lcm (a; b) = aj. it follows that lcm (a; b) =
gcd (a; b) ij. For the second part, if lcm (a; b) = gcd (a; b), then ij = 1, so
i and j are 1, so lcm (a; b) = a = gcd (a; b). By symmetry, lcm (a; b) = b =
gcd (a; b). So a = b.
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