If it is an arithmetic sequence, then if you subtract
the first term from the second (69 - -31)
and the second term from the third (169 - 69)
and the third term from the fourth (269 - 169)
you will get the same answer every time ...
Are the answers to these three subtraction problems the same number?
If so, it's an arithemetic sequence; if they are not all the same answer then it isn't an arithmetic sequence ...
If it is an arithmetic sequence, then if you subtract
the first term from the second (69 - -31)
and the second term from the third (169 - 69)
and the third term from the fourth (269 - 169)
you will get the same answer every time ...
Are the answers to these three subtraction problems the same number?
If so, it's an arithemetic sequence; if they are not all the same answer then it isn't an arithmetic sequence ...