Explanation:
to add to the explanation :
an arithmetic sequence is defined as
an = an-1 + d
so, every term of the sequence is created by adding the same constant d (the common difference) to the previous term.
therefore, you find the common difference by finding the difference of 2 sequential terms.
like here e.g.
a3 = 38
a4 = 53
53 - 38 = 15
bingo !
and to verify that it truly is an arithmetic sequence, we can quickly also check the differences between the other pairs. yes, they are all 15.
be prepared for a trap like
1 8
2 23
3 38
5 68
notice the sudden jump in x ? suddenly x is increased by 2.
that just means we compare
an and an-2 instead of an-1.
so, how is related to an-2 ?
an = an-1 + d = (an-2 + d) + d = an-2 + 2d
and so we would get
a3 = 38
a5 = 68
a5 - a3 = 2d = 30
d = 15
so, don't get confused by such jumps or gaps.