Question

A what is a letter or symbol that represents some unknown value

Answer 1

unknown values can be taken as anything , for example : the most common is x for unknown values and sometimes, for angles we can take θ(theta).

it is totally upto you!

Answer 2

Variables are arbitrary unknown values that are used to represent a value that isn't constant. They are commonly referred to as 'placeholder' values. These are commonly also referred to as parameters when dealing with co-ordinate geometry.