In: Computer Science
For the following exercises, find the specified term for the geometric sequence, given the first four terms.
an = {−1, 2, −4, 8, ...}. Find a12.
Consider the following geometric sequence:
{=1, 2, -4, 8} ...... (1)
The first term of above sequence is a1 = -1.
Common ratio of the above sequence will be obtained by dividing its second term by the first term,
r = a2 ÷ a1
= 2 ÷ (-1)
= -2
Use the formula for nth term of a geometric sequence,
an = a1 × r(n-1) ...... (2)
Substitute a1 = 1, n = 12 and r = -2 in formula (1) and compute 12th term of the sequence,
a12 = (-1) × (-2)(12-1)
= (-1) × (-2)11
= (-1) × (-2048)
= 2048
Therefore, 12th term of the above geometric sequence is a12 = 2048.
Therefore, 12th term of the above geometric sequence is a12 = 2048.