Question

What is formula for following geometric sequence -5,-10,-20,-40

Answer 1

x = -5 * (2)^(n-1)

Explanation:

Answer 2

x = -5 * (2)^(n-1)

Explanation:

If we look a the supplied n elements of the sequence, and we divide each of them by -5.

1 = -5, -5/-5 = 1

2 = -10, -10/-5 = 2

3 = -20, -20/-5 = 4

4 = -40, -40/-5 = 8

We realize that we have all the powers of 2 there.

So the formula will start with -5 (the first element), then multiplied by a power of 2.

`x = -5 (2)^(n-1)`

We can verify for the 4th element, where n = 4:

`x = -5 (2)^(n-1) = -5 (2)^(4-1)  = -5 (2)^(3)  = -5 * 8 = -40`