Question

Find a formula for the 4th term of the following G.P.S a) 1, 2,4......... b)50, 20, 8......... Pls explain very well.. Thank you.

Answer 1

The nth term is (n-1)^2

The 4th term is (4-1)^2 = 9

Answer 2

see below

Explanation:

The formula for a geometric sequence is

an = a1 * r^ (n-1) where a1 is the first term and r is the common ratio

The common ratio is found by taking the second term and dividing it by the first term

a) 1, 2,4.........

a1 = 1

r = 2/1 = 2

an = 1 * ( 2) ^ (n-1)

Let n = 4

a4 = 1 * 2^ (4-1) = 1 * 2^3 = 8

b)50, 20, 8.........

a1 = 50

r = 50/20= 5/2

an = 50 * ( 5/2) ^ (n-1)

Let n = 4

a4 = 50 * (5/2)^ (4-1) = 50 * (5/2)^3 = 3125/4