Remember those days when you learned the sequence of numbers, for example :

**1, 3, 5, 7,…….** or **15, 10, 5, 0, -5, -10,……** or **-1.4, -0.7, 0, 7,…..**

For those who are going into Form 5 in the next few days, you will be looking at something similar soon. Unlike the concepts that we have learned in the early schooling days, this time around the concentration will be a little more detail and comprehensive.

Before we go even further into this chapter, let us identify certain terms to describe the content of this sequence number of which will also be summarised as the basic formulae of this chapter.

- The sequence of numbers that progress constantly is known as Arithmatic Progression (AP); for example: 1,3,5,7,…is an AP sequence as it has a constant difference….. (7-5) = (5-3) = (3-1) = 2

- An ascending AP will have a
**positive difference** while a descending AP will have a **negative difference**

- The first number on the AP sequence will be known as
**a** or **term 1** while the difference will be known as **d**; as such the AP sequence numbers can also be described as **a, a+d, a+2d, a+3d,…..**

- The sum of an AP sequence number means the sum of the numbers listed on the AP sequence; for example
**S₃ = T₁ + T₂ + T₃ = a + (a+d) + (a+2d) = the sum of all the first 3 terms**

Let us study 2 of the examples shown above and try to understand the actual concept behind all these sequence numbers. InsyaAllah understanding the basic concept of the sequence number will help you to master this chapter with great ease.

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