What is relations among the sides of a triangle?

Answer 1

There are several. Some of the more common ones are:

Any two sides of a triangle must be larger than the third.

If a, b and c are the lengths of the sides and A, B and C are the angles oppostie these sides, then

sin(A)/a = sin(B)/b = sin(C)/c

a2 = b2 + c2 - 2bc*Cos(A)