So, substituting for :
Plugging these into (1) gives:
Each of these results gives the same set of numbers: .
You could use the ones you suggest here, since you have written 4 numbers with a common difference (in this case ). Or, more staightforwardly, you could do as I have in question (1), and assume that the first is , common difference , and write them as .2) Also, what the general idea for assuming "any set of numbers" required by the condition?
For eg. if the problem demands four numbers, the assumed ones are (a-3d),(a-d),(a+d),(a+3d). Where "d" is the common difference.
How do you frame them?
Either way, you'll have two unknowns, and you'll therefore need to be able set up two equations in order to solve a particular problem.