Find the lengt of side c ofa triangle. if sides ais 1 and side bis 20?

The pyhtagorean states that to find c, use the eqaution, . 
Substitute 1 for a  and 20 for b;
c^2=1+400
c^2=401
c=
which is about 20.02
Hope this helps.

If the lengths of two of the sides of a triangle are  1  and  20,
then the third side can be any length between  19  and  21.

It's not possible to calculate it exactly until we know
more information about the triangle.

For example, if the triangle is a right triangle,
then the length of the third side is

either    √401  =  roughly  20.025

or          √399  =  roughly  19.975 .