what is the rule for use of letters in place of numbers
I am not aware of any rule. Different unknowns get different letters. In physics and engineering, the letter chosen is often the initial of the quantity it represents (such as "r" for "radius"). Greek letters are often chosen for angles or continuous variables in Field Theory.
The exception are perhaps constants where there is a generally accepted set of symbols to replace the numbers such as pi, R, e, G, c and so on.
The rule for using letters in place of numbers is called algebraic notation. In algebra, letters are used to represent unknown numbers or variables. This allows us to solve for unknown quantities and work with general mathematical relationships.
To use letters in place of numbers, you can follow these steps:
1. Choose a letter to represent the unknown quantity. Common choices include x, y, and z, but any letter can be used.
2. Define the problem or equation using the chosen letter. For example, if you need to find the value of a number that is 5 more than another number, you can represent the unknown number as x and write the equation as x = x + 5.
3. Solve the equation by finding the value of the letter that makes the equation true. This is typically done by performing algebraic operations such as addition, subtraction, multiplication, and division to isolate the variable.
By using letters instead of specific numbers, algebra allows us to work with general mathematical concepts and find solutions that are applicable to a wide range of situations.