The formula for any arithmetic sequence is an = a1 + d(n - 1), where an represents the value of the nth term, a1 represents the
value of the first term, d represents the common difference, and n represents the term number. What is the formula for the sequence -15, -11, -7, ...? an = -15 + (-4)(n - 1) an = -15 + (n - 1) an = 4 + (-15)(n - 1) an = -15 + 4(n - 1)
To figure this out you would have to just lug in all of the numbers into the equation to find the correct answer.
So first we would plug in the first number for a1 which will be -15. Then we would put in d or the common difference (the number of numbers in between the 2 numbers). So d in this case would be positive 4 since every single time the number is being added to 4.
Now we can solve, so we would just find all of the choices that do not have +4 because that will be wrong. Automatically you can see that choices 1 and 2 are wrong and 3 also looks wrong and the only one that matched our formula is 4 and 4 is the correct answer.
