I have a question about using articles.
Consider the following sentence.
"The/a right side of a rectangle can be found ..." "The perimeter of a rectangle may be / is found by"
The question is : Should I use the definite or the indefinite article in this case ? I am describing not a particular rectangle, but any rectangle, for example this sentence could describe the formula to calculate the position of the right corner of a rectangle.
Please explain this rule, and also it is very important for me to know if I've made any mistakes in using articles in the question.
Thank you so much.