Two rods each of length `L_(2)` and coefficient of linear expansion `alpha_(2)` each are connected freely to a third rod of length `L_(1)` and coefficient of expansion `alpha_(1)` to form an isoscles triangle. The arrangement is supported on a knife-edge at the midpoint of `L_(1)` which is horizontal. what relation must exist between `L_(1)` and `L_(2)` so that the apex of the isoscles triangle is to remain at a constant height from the knife edge as the temperature changes ?