Arithmetic and Geometric Sequences

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This Higher GCSE scheme of work unit covers sequences in Year 10. It finds a linear nth term, then a picture-sequence rule, generates terms from a formula, finds a quadratic nth term, writes a geometric nth term, and finishes with a recurrence.

Students have to treat n as a position, not as the next jump, and they have to use ar^(n−1) rather than ar^n. Those two distinctions are what later iteration, proof and compound-growth items all use.

What Success Looks Like

A student who has secured this unit:

  • Finds a linear nth term from a constant first difference and tests it on the first term.
  • Finds an nth term from a growing diagram and generates terms from a given formula.
  • Finds a quadratic nth term that matches the second difference and the first term.
  • Writes a geometric nth term as ar^(n−1) and generates terms from a recurrence without mixing the two.

Prerequisite Knowledge

Students should be secure with the following before beginning this unit.

  • Substitute a positive integer into a one-step expression such as 3n + 1.
  • Continue a number pattern that increases by a constant.
  • Collect like terms in a linear expression.
  • Generate the first few terms from a given nth term such as 4n − 1.

Key Mathematical Ideas

Position, not jump

n is the term’s place in the list. Writing n + 5 for 3, 8, 13, 18 describes the difference, not the position-to-term rule.

Second difference

A constant first difference means linear. A constant second difference means quadratic. The n² coefficient is half of that second difference.

Common ratio

A geometric sequence multiplies by a constant r. Its nth term is ar^(n−1), so term 1 is a, not ar.

A rule from the last term

A recurrence gives u_(n+1) from u_n. It is not an nth term. You still need a starting value.

Working Mathematically

Fluency

  • Find the nth term of 7, 11, 15, 19 and the 25th term.
  • Find the nth term of 4, 7, 12, 19 and the 10th term.
  • Generate the first five terms of u_(n+1) = 3u_n + 1 with u_1 = 2.

Reasoning

  • Explain why 3, 8, 13, 18 is 5n − 2 and not n + 5.
  • Show that first differences 4, 6, 8, 10 cannot belong to a linear sequence.
  • Say why a geometric formula that uses n rather than n − 1 fails when n = 1.

Common Misconceptions with Sequences

MisconceptionTeaching focus
Treating n as the next term, so 3, 8, 13, 18 is written as n + 5.Ask for the 1st term from the proposed formula. If it is not 3, n has been used as a jump.
Using ar^n for a geometric sequence instead of ar^(n−1).Substitute n = 1 into the proposed formula. The output has to be the first term a, not ar.
Stopping at the n² piece of a quadratic and ignoring the leftover linear sequence.Take the quadratic part away from each term and find the nth term of the list that remains.
Mixing a recurrence with an nth term, so u_(n+1) = 2n + 1 is treated as the 2n + 1 sequence.Ask whether the rule uses n or the previous term before any values are generated.
Taking the common ratio as the first difference of a geometric list.Divide consecutive terms, rather than subtracting them, on the first geometric examples.

Differentiation

Additional support

  • Write 1, 2, 3, 4, 5 as a header row above the sequence before a formula is attempted.
  • Provide the first two diagrams with the extra edges already in a second colour.
  • Fill the first-difference row of a table and leave the second-difference row blank for students to complete.

Additional challenge

  • A stick pattern adds a hexagon each time. Find how many sticks are in the 40th pattern.
  • A geometric sequence has third term 24 and common ratio −1/2. Find the first term and the fifth term.
  • A quadratic sequence begins 2, 8, 18, 32. Find the nth term and the first term that exceeds 300.

Teach This Unit with Mr Mathematics

Every lesson in this unit is already built. A Mr Mathematics membership gives you the presentation, worksheet, question generator and video tutorial for all 6 lessons, alongside the full Key Stage 3, GCSE, IGCSE and A-Level libraries.

A school membership covers every teacher in your department, so the whole scheme of work is resourced from one subscription.

Arithmetic and Geometric Sequences Lessons

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