To find the limits of the third side, we use the triangle inequality, which says that "the sum of lengths of any two sides must be less the third side, AND must be greater than the difference of these two sides".
If 8 & 12 are the shortest legs, then the third side must be less than (8+12)=20 ft. , and must be more than (12-8)=4 ft.
So the limits of the third side x of this triangle is
4 < x < 20
