Find the 13th term of the sequence 8, -16, 32, -64,
a)32768 b)none of these c)-65536 d)-16384

This is a geometric sequence. To find the common ratio, divide the second term by the first term.
-16/8 = -2

an = a1 * r^(n-1)
n = term to find = 13
a1 = first term = 8
r = common ratio = -2

a13 = 8 * -2^(13 - 1)
a13 = 8 * -2^12
a13 = 8 * 4096
a13 = 32768 <===