Question

Find the nth term of this number sequence 15, 12, 9, 6, .... ​

Answer 1

Answer: -3n + 18

Explanation:

When examining the sequence, you can see that every time, it decreases by 3, therefore the first part of your rule should be:

-3n

(Extra note to wow your maths teacher! This is a linear sequence as it decreases/increases by the same amount each time.)

Next, you will need to find the number before the given sequence. As it decreases by 3 every time, to find the number before you need to increase by 3. Therefore, the next part you need is + 18.

Leave a comment if you don't understand anything :)

Answer 2

• The nth term is 18 - 3n

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Given sequence:

• 15, 12, 9, 6, .... ​

This is an AP, with the first term a = 15 and common difference of - 3, as it is decreasing by 3.

Use nth term equation for an AP:

• t(n) = a + (n - 1)d
• t(n) = 15 + (n - 1)(-3) = 15 - 3n + 3 = 18 - 3n