### Theory:

In any triangle, the difference in the length of any two sides of a triangle is always lesser than the third side

In here, the inequalities are as follows:
$$a - b < c$$

$$b - c < a$$ and

$$c - a < b$$
Example:
Consider the triangle$$ABC$$ whose sides measures are $$AB = c = 3 cm, BC = a = 4 cm$$ and $$AC = b =5 cm$$.

Let's check the triangle inequality for the triangle $$ABC$$,
$$a - b = 4 - 5 = -1 < 3 = c$$
$$b - c = 5 - 3 = 2 < 4 = a$$ and
$$c - a = 3 - 4 = -1 < 5 = b$$

Important!
Suppose $$a, b$$ and $$c$$ are the sides of a triangle with $$a$$ and $$b$$ are known sides and $$c$$ is unknown.  Let's use the triangle inequality to find the third side $$c$$.  As the sum of two sides$$(a+b)$$ of a triangle is always greater than the third side$$(c)$$, and difference of two sides$$(a-b)$$ of a triangle is always lesser than the third side$$(c)$$, the length of the third side$$(c)$$ must lie between difference$$(a-b)$$ and sum$$(a+b)$$.