How to keep one variable constant with other one changing with row in excel

Question

Lets say I have one cell A1, which I want to keep constant in a calculation. For example, I want to calculate a value like this:

=(B1+4)/(A1)

Answer 1

You put it as =(B0+4)/($A$0)

You can also go across WorkSheets with Sheet1!$a$0

Answer 2

Placing a $ in front of the row value to keep constant worked well for me. e.g.

=b2+a$1