In Year 9, arithmetic sequences are an important idea in math. Here are some key points to know:

• What It Is: An arithmetic sequence is a list of numbers where each number either goes up or down by the same amount each time. This amount is called the common difference (we use the letter $d$ to represent it).

• How to Write It: We can find the $n^{th}$ number in the sequence using this formula: $a_n = a_1 + (n - 1)d$. Here, $a_1$ is the first number in the sequence.

• An Example: Let’s look at the sequence 3, 7, 11, 15. In this case, the common difference $d$ is 4 because if you add 4 to 3, you get 7. If you add 4 to 7, you get 11, and so on.

Knowing these points helps you understand sequences and series better!