In: Math
Write a recursive formula for the arithmetic sequence 0, −1/2 , −1, −3/2 , … , and then find the 31st term.
Consider the arithmetic sequence,
0, -1/2, -1, -3/2 …
Use the recursive formula for an arithmetic sequence,
a1
an = an-1 + d ...... (1)
First term of the sequence is a1 = 0
Compute common difference as follows:
d = a2 – a1
= 1/2 – 0
= -1/2
Substitute d = -1/2 in the formula (1) an = an-1 + d and simplify,
an = an-1 – 1/2
Therefore, recursive formula for given arithmetic sequence is,
a1 = 0
an = an-1 -1/2, n ≥ 2
Consider the explicit formula for the nth term of an arithmetic sequence as
an = a1 + (n – 1)d …... (2)
Compute 31st term of the given sequence, substitute a1 = 0, n = 31 and d = -1/2 in equation (2) an = a1 + (n – 1)d and simplify,
a31 = a1 + (31 – 1)d
= 0 + 30 × (-1/2)
= 0 - 15
Therefore, 31st term of the given sequence is a31 = -15.
Therefore, 31st term of the given sequence is a31 = -15.