Answer:
5 < third side < 21
Step-by-step explanation:
Let the length of the third side be [tex]x[/tex].
We know that [tex]x[/tex] must be smaller than the sum of the lengths of the two other sides. Thus [tex]x < 21[/tex].
We also know that
[tex]x+8 > 13\\x > 5[/tex]
And
[tex]x+13 > 8\\x > -5[/tex]
So, in conclusion, [tex]5 < x < 21[/tex].