Skip to content

Class 9 Maths · Chapter 8 · NCERT (iemh108)

Predicting What Comes Next: Exploring Sequences and Progressions: common misconceptions

See the tempting mistake students make in Predicting What Comes Next: Exploring Sequences and Progressions, why it feels right at first glance, and a worked visual example that shows the right idea.

The tempting mistake & worked visual

Does an increasing sequence always add the same number?

Wrong

2, 4, 8, 16 is arithmetic because every term is larger than the previous one.

Why it feels right. The regular increase looks like equal steps.

Right

An arithmetic progression has a constant difference. A geometric progression has a constant ratio.

Worked example

Term 12
Term 24
Term 38
Term 416
Term 532
Each term doubles. The rising heights make the changing differences visible.
  1. The differences are 2, 4 and 8, while the ratios are all 2.
  2. The terms double, so the pattern gives 32 as the next term.

If your child says… (8 concept checks in this chapter)

As you work through Predicting What Comes Next: Exploring Sequences and Progressions, listen for these everyday misunderstandings and use the prompt alongside each one.

1. Sequences and terms See how to teach this step →

If your child says… “The little 2 means the value is 2.”

Ask together
Before the swap, which value sits below position 2?
What’s true
The subscript chooses the position; the entry there supplies the value.
2. Explicit Rule for a Sequence See how to teach this step →

If your child says… “A number can be the 94.6th term.”

Ask together
Which entry would you point to when counting reaches 94.6?
What’s true
Positions count entries, so fractional positions cannot name terms.
3. Recursive Rule for a Sequence See how to teach this step →

If your child says… “For term three, I add 3 to the position number.”

Ask together
Which value did we find for the second term?
What’s true
The rule uses the previous term's value, starting from the given first value.
4. Arithmetic Progressions See how to teach this step →

If your child says… “The fourth term needs four additions.”

Ask together
How many extensions did we make after building the first square?
What’s true
The first term is already present, so reaching the fourth takes three additions.
5. Arithmetic Progressions: taxi fare See how to teach this step →

If your child says… “The booking fee is the first term.”

Ask together
Has the taxi covered a kilometre when we record the first term?
What’s true
The first listed fare already includes one kilometre's charge.
6. Sum of the First n Natural Numbers See how to teach this step →

If your child says… “One triangle contains the whole rectangle's total.”

Ask together
How many copies of the original staircase fill the rectangle?
What’s true
The rectangle holds two equal triangular totals, so either triangle contains half.
7. Geometric Progressions See how to teach this step →

If your child says… “I cube both the starting 3 and the multiplier 2.”

Ask together
How many times does 3 appear in the expanded first line?
What’s true
Only the multiplier repeats, so only it receives the power.
8. Visualising a GP See how to teach this step →

If your child says… “Enough halvings will make the value exactly zero.”

Ask together
What happens when you halve any positive value that still remains?
What’s true
Every finite number of exact halvings leaves a positive value.

Use this chapter three ways