In: Math
Write a recursive formula for the arithmetic sequence −2, −7/2 , − 5, −13/2 , … and then find the 22nd term.
Consider an arithmetic sequence,
-2, 7/2, -5, -13/2, …
Use the recursive formula of an arithmetic sequence,
a1
an = an-1 + d, n ≥ 2...... (1)
First term of the given sequence is a1 = -2
Common difference will be computed as follows:
d = a2\r\n– a1
= (-7/2) – (-2)
= -3/2
Substitute d = -3/2 in and simplify,
an = an-1 – 3/2
Therefore, recursive formula for given arithmetic series is,
a1 = -2
an = an-1 -3/2, n ≥ 2
Compute 22nd term of the given sequence using explicit formula of an arithmetic sequence,
an = a1 + (n – 1)d ...... (2)
Substitute a1 = -2, n = 22 and d = -3/2 in (2) and simplify,
a22 = -2 + (22 – 1)(-3/2)
= -2 + 21 × (-3/2)
= -2 – 63/2
= -67/2