New Zealand
Level 7 - NCEA Level 2

# Introduction to Arithmetic Progressions

Lesson

Recall that an ordered list of numbers, separated by commas, is called a progression or sequence, and when a pattern is detectable in a progression, a generating rule can often be established that enables us to find any term in the sequence.

Arithmetic progressions start with a first term and then either increase or decrease by a constant amount called the common difference. We denote the first term by the letter $a$a and the common difference by the letter $d$d.

The progression $-3,5,13,21,\ldots$3,5,13,21, is an arithmetic progression with $a=-3$a=3 and $d=8$d=8. On the other hand, the progression $1,10,100,1000,\ldots$1,10,100,1000, is not arithmetic because the difference between each term is not constant.

Consider the arithmetic sequence $-4,3,10,17,...$4,3,10,17,... We know that it is arithmetic because $3-\left(-4\right)=10-3=7$3(4)=103=7 and so with the common difference of $7$7, the next three terms are $24,31$24,31, and $38$38.

We also can develop a rule that generates an arithmetic progression. For example, take the rule given by  $t_n=80-7n$tn=807n. We can use this formula to begin writing down the sequence. The first term, or $t_1$t1 is found by substituting $n=1$n=1 into the formula, so $t_1=80-7\times1=73$t1=807×1=73. Similarly $t_2=80-7\times2=66$t2=807×2=66. We can find any term this way.

For example, the first negative term in the sequence is given by $t_{12}=80-7\times12=-4$t12=807×12=4.

This applet will allow you to explore the geometrical features of an arithmetic progression, change the values of $a$a (the initial term) and $d$d (the common difference). What shape is the line?

#### Practice questions

##### QUESTION 1

Study the pattern for the following sequence, and write down the next two terms.

1. $6$6, $2$2, $-2$2, $-6$6, $\editable{}$, $\editable{}$

##### QUESTION 2

Study the pattern for the following sequence.

$280$280, $230$230, $180$180, $130$130 ...

1. State the common difference between consecutive terms.

##### QUESTION 3

A diving vessel descends below the surface of the water at a constant rate so that the depth of the vessel after $1$1 minute, $2$2 minutes and $3$3 minutes is $50$50 metres, $100$100 metres and $150$150 metres respectively.

1. By how much is the depth increasing each minute?

2. What will the depth of the vessel be after $4$4 minutes?

3. Continuing at this rate, what will be the depth of the vessel after $10$10 minutes?

### Outcomes

#### M7-3

Use arithmetic and geometric sequences and series

#### 91258

Apply sequences and series in solving problems