Answer:
triangle inequality: the sum of the length of two sides of a triangle must always be greater than the length of the third side.
we know 2 sides whose sum = 5+8 = 13cm
the 3rd side must be <13 included
the length of the third side must be less than 13 cm, i.e. between 1 and 13 (it cannot be equal to 0 because in this case it does not exist)