##a_n = 25-10(n-1)##

An is a sequence in which there is a constant difference between successive terms. Given an arithmetic sequence with initial term ##a_1## and a constant difference ##d## we can write the ##n^th## term as ##a_n = a_1+d(n-1)##

In this case as each term is ##10## less than the previous term the sequence is potentially an arithmetic sequence with initial term ##25## and constant difference ##-10## meaning we would have ##a_n = 25 -10(n-1)##