if he is referring to a cable that sags and barely touching the ground, then it's not what i'm thinking.

i am thinking of a 'Stay' cable tied at the apex of towers and on a collinear point on the ground between 2 towers which must be defined by maxima/minima to determine the least length.

yeah, as what DMS said, if you want to be precise on the answer, you must give the cable properties, temperature, wind factors etc. etc. etc... then we will give you your answers.

i'll drop this one, due to ambiguity and let others do the math... ;)