Extremely easy math, but need expression

[Joel DB]

Okay,
This could be the easiest math question on here... but I'm going to ask the dumb question anyway. I have several coordinate points to solve for, and in each case I subtract the same amount from the previous "x" to get the next "x" value (X2...3...4...5...). The only point where I subract a different value is X1. The rest have a constant rate of change (derivative?). Example:

1.) X1 = (some vaule) – 30’
2.) X2 = (X1 – 60’), X3 = (X2 – 60’), X4 = (X3 – 60’), X5 = (X4 – 60’), X6 = (X5 – 60’)

There has to be a simple mathematical formula, other than this sloppy one. Any help would be great.

 

[Mark44]

See http://en.wikipedia.org/wiki/Arithmetic_progression.

 

[rama]

like Mark44 said this is an arithmetic progression
Here the difference between two terms(of the sequence) is equal
T_n=a+(n-1)d
where a is the first term and d is the difference between two terms(d=T₂-T₁)