S is the surface z = xy in Euclidean 3space.
Find all straight lines lying in S.
One set is the intersection of the planes y=k;z=kx for any real k.
Another is x=k;z=ky for all real k.
Expressed parametrically:
Set one: y=k;x=t;z=kt
Set two: y=t;x=k;z=kt
That there are no others is shown by the fact that for the line to be straight while both x and y vary, y must be in the form ax+b. Then the equation for z becomes z=x(ax+b) = ax˛+bx, but if a is not zero, this is not linear, so a must be zero (so long as x is not a constant) and y a constant. Interchange x and y in the above for the other way of looking at why not both x and y can vary.

Posted by Charlie
on 20040223 08:33:34 