Question

If the starting number is 30 and the rule is "minus five", what is the formula and what is the 20th number in the sequence?

Answer 1

y = -5x + 30, 20th number is -70.

Explanation:

All you have to do is look at how much the equation is growing by and what the starting number is for the equation. to find the 20th figure, you plug 20 in for x and get -70.

Answer 2

an = 30 -5(n-1) OR an = 35 - 5n

a20 = -65

Explanation:

The formula for an arithmetic sequence is

an =a1 +d(n-1)

where a1 is the first term

d is the common difference

n is the term number

Substituting what we know

an = 30 -5(n-1)

We can rewrite this by distributing the 5

an = 30 -5n +5

= 35 - 5n

The 20th term is n=20

a20 = 30 - 5(20-1)

= 30 -5(19)

= 30 - 95

= -65