Question

How do I find the third side of a triangle?

Answer 1

if it is a right angle triangle then you can find it by using Pythagoras theorem
other wise perimeter of the triangle would be given.
all over it just depends on question

Answer 2

Third side of a triangle can determined through following ways.

1st approach:  

When the triangle given possess a right angle (90°), then side facing it is called hypotenuse which is larger than the other two sides. If ‘a’ is the hypotenuse and ‘b’ and ‘c’ are the other two sides, then using Pythagoras theorem any unknown side can be determined knowing other two sides.

It is given by,

$a^2=b^2+c^2$

2nd approach:  

When the triangle doesn’t possess a right angle and knowing the angle subtended by two sides, then

$a^2=b^2+c^2-2bc\ cos\ cos\theta$  

This approach is called Law of cosine.

Or

When the triangle doesn’t possess a right angle and knowing any two sides and an angle, then

$\frac{a}{\sin\ sin\ A } =\frac{b}{\sin\ sin\ B } =\frac{c}{\sin\ sin\ C }$  

This approach is called Law of sine.