Consider two numbers X and Y, whose GCD is g. Then X = gx and Y = gy, and x and y are relatively prime. Then the LCM of X and Y is gxy.
The conditions of the problem require LCM(X,Y) + GCD(X,Y) = X+Y, or:
gxy + g = gx + gy and (dividing by the nonzero g)
xy + 1 = x + y
subtracting y + 1 from both sides and collecting terms gives
y(x1) = (x1)
if x = 1, then this is true for all y, otherwise we can divide by (x1) and conclude that y = 1. Either way, at least one of x and y = 1.
Assume that y = 1. Then Y = gy = g and X = gx and so Y divides X. Similarly, if we take x = 1 then X = g and X divides Y, and if x = y = 1 then X = Y = g and X and Y each divide the other.
In all cases, then, if the equation holds then one of the two numbers divides the other.

Posted by Paul
on 20160815 13:30:32 