Arithmetic Sequence and Arithmetic Series
Constant first difference. Linear general term. Level 1 to 3.
Recognising the three types
- First differences are constant.
- Subtract consecutive terms.
- \(d = T_n - T_{n-1}\)
- The general term is linear.
- Example: \(3;\;7;\;11;\;15;\ldots\) with \(d = 4\).
- Second differences are constant.
- You need at least 4 terms to confirm it.
- The first differences form an arithmetic sequence.
- The general term is quadratic.
- Example: \(6;\;12;\;22;\;36;\ldots\)
- There is a constant ratio between terms.
- Divide consecutive terms.
- \(r = T_n \div T_{n-1}\)
- The general term is exponential.
- Example: \(2;\;6;\;18;\;54;\ldots\) with \(r = 3\).
Definition and common difference
A sequence where the difference between any two consecutive terms is constant. That constant is the common difference \(d\).
General form: \(\;a,\quad a+d,\quad a+2d,\quad a+3d,\quad\ldots\)
The sequence \(3;\;-1;\;-5;\;\ldots\) from a September 2024 Mpumalanga pre-trial paper has \(d = -4\). It is still an arithmetic sequence because the difference is constant. Always check the sign of \(d\) before substituting.
Source: G12 Maths P1 Mpumalanga Pre-Trial Sept 2024, Q2.2
An arithmetic series has its \(19\)th term equal to \(11\) and its \(31\)st term equal to \(5\). Find \(a\) and \(d\).
- 1Write the two term equations: \(T_{19} = a + 18d = 11\) and \(T_{31} = a + 30d = 5\).
- 2Subtract them: \(12d = -6 \Rightarrow d = -\tfrac{1}{2}\).
- 3Substitute back: \(a + 18\left(-\tfrac{1}{2}\right) = 11 \Rightarrow a = 20\).
- 4So the sequence starts \(20;\;19.5;\;19;\;\ldots\)
Source: 2025 G12 March Khayelitsha PLC, Q1.1
General term: \(T_n = a + (n-1)d\)
\(a\) is the first term \((T_1)\), \(d\) is the common difference, and \(n\) is the term position.
Show derivation of \(T_n = a + (n-1)d\)
Sequence: \(4;\;10;\;16;\;\ldots\), so \(a = 4\) and \(d = 6\).
- 1\(T_n = 4 + (n-1)(6) = 6n - 2\)
- 2\(T_{50} = 6(50) - 2 = \mathbf{298}\)
Source: G12 Maths P1 NW Sept 2024, Q2.2
- 1From the previous example, \(a = 20\) and \(d = -\tfrac{1}{2}\).
- 2Use \(S_n = \dfrac{n}{2}[2a + (n-1)d]\).
- 3\(S_{31} = \dfrac{31}{2}[2(20) + 30(-\tfrac{1}{2})] = \dfrac{31}{2}(25) = \mathbf{387.5}\)
Source: 2025 G12 March Khayelitsha PLC, Q1.2
Calculate the sum of all the integers from \(100\) to \(300\) that are multiples of \(4\).
- 1Write the arithmetic series: \(100 + 104 + 108 + \cdots + 300\).
- 2Find the number of terms: \(300 = 100 + (n-1)4 \Rightarrow 4n = 204 \Rightarrow n = 51\).
- 3Now sum: \(S_{51} = \dfrac{51}{2}(100 + 300) = \mathbf{10\,200}\).
Source: 2025 G12 SACAI P1 May_June, Q2.2
Sum formula: \(S_n = \tfrac{n}{2}[2a + (n-1)d]\)
Use \(S_n = \frac{n}{2}(a + l)\) only when the last term \(l\) is given or easy to find.
Full proof: Arithmetic series sum formula
Do not use the formula you are proving inside the proof itself. Use only \(T_n = a + (n-1)d\) and the structure of the sum.
For the arithmetic sequence \(3;\;-1;\;-5;\;\ldots\;-85;\;-89\), calculate the sum of all negative terms.
\[S_{22} = \frac{22}{2}(-3 + -89) = 11(-92) = \mathbf{-1\,012}\]Source: G12 Maths P1 Mpumalanga Pre-Trial Sept 2024, Q2.2.2
\(5 + 3 + 1 + \cdots = -216\), so \(a = 5\) and \(d = -2\).
- 1\(-216 = \tfrac{n}{2}[2(5) + (n-1)(-2)]\)
- 2\(-432 = 12n - 2n^2 \Rightarrow 2n^2 - 12n - 432 = 0 \Rightarrow n^2 - 6n - 216 = 0\)
- 3\((n-18)(n+12) = 0 \Rightarrow n = 18\), since \(n\) must be a positive natural number.
- 4So 18 terms sum to \(-216\).
Given \(S_n = 3n^2 - 2n\). Find \(T_9\).
- 1\(S_9 = 3(81) - 18 = 225\)
- 2\(S_8 = 3(64) - 16 = 176\)
- 3\(T_9 = S_9 - S_8 = 225 - 176 = \mathbf{49}\)
Key identity: \(\boldsymbol{T_n = S_n - S_{n-1}}\) for \(n \geq 2\), and \(T_1 = S_1\).
The formula is \(a + (n-1)d\), not \(a + nd\). The bracket matters because the first term contains zero copies of the common difference.
1. Given \(5;\;9;\;13;\;17;\;\ldots\), find the general term and then state which term equals 217.
2. The sum \(22 + 28 + 34 + \cdots\) equals 1870. Find the number of terms.
3. \(3x+1;\;2x;\;3x-7\) are the first three terms of an arithmetic sequence. Find \(x\).
4. \(S_n = 3n^2 - 2n\). Determine \(T_9\).
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Quadratic Sequence
Constant second difference. Quadratic general term. Level 2 to 3.
Definition and difference tables
A sequence where the second differences are constant. The first differences form an arithmetic sequence.
Sequence: \(13;\;27;\;45;\;67;\;\ldots\)
| Position | \(T_1\) | \(T_2\) | \(T_3\) | \(T_4\) |
|---|---|---|---|---|
| Terms | 13 | 27 | 45 | 67 |
| First differences | 14 | 18 | 22 | |
| Second differences | 4 | 4 |
The second differences are constant, so this is a quadratic sequence.
Next first difference: \(26\), so the next term would be \(93\).
Source: 2024 G12 CT March Sedibeng West, Q2.1
Finding \(T_n = an^2 + bn + c\)
Solve in order. Find \(a\) first, then \(b\), then \(c\).
For \(13;\;27;\;45;\;67;\;\ldots\), the second difference is 4.
- 1\(2a = 4 \Rightarrow a = 2\)
- 2\(3a + b = T_2 - T_1 = 14 \Rightarrow 6 + b = 14 \Rightarrow b = 8\)
- 3\(a + b + c = T_1 = 13 \Rightarrow 2 + 8 + c = 13 \Rightarrow c = 3\)
- 4So \(T_n = 2n^2 + 8n + 3\). Checking \(n = 3\) gives \(45\), which matches the sequence.
Source: 2024 G12 CT March Sedibeng West, Q2.1.1
Use the quadratic rule \(T_n = 2n^2 - 2n + 3\).
- 1Set the term rule equal to the target value: \(2n^2 - 2n + 3 = 4\,903\).
- 2\(2n^2 - 2n - 4\,900 = 0 \Rightarrow n^2 - n - 2\,450 = 0\).
- 3Factorise: \((n - 50)(n + 49) = 0\).
- 4Reject the negative root and keep \(n = 50\).
- 5So \(4\,903\) is the 50th term.
Source: 2025 G12 Free State P1 June, Q2.1.3
After finding the rule, substitute \(n = 1\), \(n = 2\), and \(n = 3\) back into \(T_n\). This short check can save the whole question if you made a sign error.
1. Sequence: \(2;\;7;\;15;\;26;\;40;\;\ldots\). Find \(T_n\), then determine which term equals 260.
2. Find \(T_6\) and \(T_n\) for \(-1;\;3;\;9;\;17;\;27;\;\ldots\)
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Geometric Sequence and Geometric Series
Constant ratio. Exponential general term. Finite and infinite sums. Level 1 to 4.
Definition and common ratio
A sequence where each term is obtained by multiplying the previous term by a constant ratio \(r\).
General form: \(\;a,\quad ar,\quad ar^2,\quad ar^3,\quad\ldots\)
The 4th term of a geometric sequence is \(6\), and the 9th term is \(0.1875\). Determine the sequence.
- 1Use \(T_9 = ar^8\) and \(T_4 = ar^3\). Dividing gives \(r^5 = \dfrac{0.1875}{6} = \dfrac{1}{32}\).
- 2So \(r = \dfrac{1}{2}\).
- 3Now \(6 = a\left(\dfrac{1}{2}\right)^3\), so \(a = 48\).
- 4The sequence is \(48;\;24;\;12;\;6;\;\ldots\)
Source: 2025 G12 SACAI P1 May_June, Q2.1
General term and finite sum
Both finite sum forms are equivalent when \(r \neq 1\). Use the version that keeps your arithmetic cleaner.
Full proof: Geometric series sum formula
The first two terms of a converging geometric series are \(8\) and \(m\), and the sum to infinity is \(12\). Determine the constant ratio.
\[12 = \frac{8}{1-r} \Rightarrow 12(1-r) = 8 \Rightarrow r = \mathbf{\frac{1}{3}}\]Source: 2025 G12 SACAI P1 May_June, Q2.3
\(2 + 6 + 18 + \cdots = 728\), so \(a = 2\) and \(r = 3\).
- 1\(728 = \dfrac{2(3^n - 1)}{2} = 3^n - 1\)
- 2So \(3^n = 729 = 3^6\), which means \(n = 6\).
Convergence and sum to infinity
Convergent series
The terms shrink toward zero, so the series adds to a finite value. This happens when \(|r| < 1\).
Example: \(8;\;4;\;2;\;1;\;\ldots\) with \(r = \tfrac{1}{2}\)
Divergent series
The terms grow or keep oscillating, so there is no finite sum to infinity.
Example: \(2;\;4;\;8;\;16;\;\ldots\) with \(r = 2\)
Sequence: \(8;\;4;\;2;\;1;\;\ldots\), so \(a = 8\) and \(r = \tfrac{1}{2}\).
Since \(\left|\tfrac{1}{2}\right| < 1\), the series converges.
\[S_\infty = \frac{8}{1-\frac{1}{2}} = \frac{8}{\frac{1}{2}} = \mathbf{16}\]The geometric series below is convergent: \(1 + \dfrac{2x-5}{2} + \left(\dfrac{2x-5}{2}\right)^2 + \cdots\)
- 1The common ratio is \(r = \dfrac{2x-5}{2}\).
- 2For convergence, \(-1 < \dfrac{2x-5}{2} < 1\).
- 3Multiply through by \(2\): \(-2 < 2x-5 < 2\).
- 4Add \(5\): \(3 < 2x < 7\).
- 5Divide by \(2\): \(\dfrac{3}{2} < x < \dfrac{7}{2}\).
Source: 2024 G12 SACAI P1 May_June, Q2.3.1
State and verify \(|r| < 1\) before using \(S_\infty\). That line is usually one of the method marks in this type of question.
1. Find the 10th term of \(3;\;6;\;12;\;\ldots\)
2. The first two terms are \((x+3)\) and \((x^2-9)\). For which \(x\) does the series converge?
3. The first four terms are \(7;\;x;\;y;\;189\) with \(r = 3\). Find \(x\) and \(y\).
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Sigma Notation
Reading, interpreting, counting, and evaluating. Level 2 to 4.
Anatomy of a sigma expression
The Greek capital sigma means "add all these terms".
The expression after \(\Sigma\). This is the term rule \(T_k\).
The value below \(\Sigma\). It tells you where to start substituting.
The value above \(\Sigma\). It tells you the last substitution value.
\(\displaystyle\sum_{k=3}^{10}\) has \(10 - 3 + 1 = \mathbf{8}\) terms.
\(\displaystyle\sum_{k=0}^{n}\) has \(n + 1\) terms.
\(\displaystyle\sum_{k=5}^{n}\) has \(n - 4\) terms.
Three-step method for every sigma question
Given \(\displaystyle\sum_{n=1}^{m}(4n-19) = 1189\), find \(m\).
- 1The first term is \(4(1)-19 = -15\), and the common difference is \(4\).
- 2Use the arithmetic-series sum formula: \(\dfrac{m}{2}[2(-15) + (m-1)4] = 1189\).
- 3Simplify: \(m(2m-17) = 1189\), so \(2m^2 - 17m - 1189 = 0\).
- 4Factorise: \((2m+49)(m-29) = 0\), giving \(\mathbf{m=29}\).
Source: 2026 G12 Limpopo CT March, Q2.3.2
Find \(n\) if \(\displaystyle \frac{5}{3}\sum_{k=1}^{n}3^{k-1} = \frac{1820}{3}\).
- 1The inner series is geometric with \(a = 1\) and \(r = 3\).
- 2\(\sum_{k=1}^{n}3^{k-1} = \dfrac{3^n - 1}{2}\).
- 3Substitute: \(\dfrac{5}{3}\cdot \dfrac{3^n - 1}{2} = \dfrac{1820}{3}\).
- 4This gives \(5(3^n - 1) = 3640 \Rightarrow 3^n = 729 = 3^6\).
- 5So \(\mathbf{n = 6}\).
Source: 2024 G12 SACAI P1 May_June, Q2.2
Calculate \(\displaystyle\sum_{p=1}^{\infty}8(4)^{1-p}\)
- 1\(T_1 = 8(4)^0 = 8\), \(T_2 = 8(4)^{-1} = 2\), so \(r = \tfrac{2}{8} = \tfrac{1}{4}\).
- 2Since \(\left|\tfrac{1}{4}\right| < 1\), the series converges.
- 3\(S_\infty = \dfrac{8}{1-\frac{1}{4}} = \dfrac{8}{\frac{3}{4}} = \mathbf{\dfrac{32}{3}}\)
\(1 + 5 + 9 + \cdots + 21\), so \(a = 1\), \(d = 4\), and \(T_k = 4k - 3\).
Find the number of terms first: \(4n - 3 = 21 \Rightarrow n = 6\).
\[\sum_{k=1}^{6}(4k-3)\]1. Evaluate \(\displaystyle\sum_{k=1}^{30}(8-5k)\)
2. Find \(m\) if \(\displaystyle\sum_{k=1}^{m}3(2)^{k-1} = 93\)
3. For which values of \(x\) will \(\displaystyle\sum_{k=1}^{\infty}(4x-1)^k\) exist?
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Exam Tips and Common Mistakes
NSC examiner insights. Cognitive levels. Complete formula reference.
Five-step strategy for every sequences question
- 1Identify the type. Check differences or ratio and write your conclusion clearly.
- 2State the known values. Write \(a = \ldots\), \(d = \ldots\), or \(r = \ldots\), as well as \(n = \ldots\), before substituting.
- 3Write the full formula. This protects your method marks.
- 4Substitute carefully. Watch brackets around \((n-1)\) and any negative values.
- 5Verify your result. If a negative root appears for \(n\), reject it by stating that \(n\) must be a positive natural number.
Seven common mistakes
This is the classic off-by-one error. The first term contains zero copies of the common difference.
The convergence check is a method mark in many Paper 1 questions.
An arithmetic sequence uses a constant difference. A geometric sequence uses a constant ratio.
You need at least 4 terms to confirm a quadratic sequence.
That final \(+1\) is what learners often miss.
A quick substitution check can save the rest of a multi-part question.
Write: "Since \(n\) must be a positive natural number, reject the negative root."
Cognitive levels: What the exam tests
Level 1: Knowledge
Recall formulas, identify the sequence type, substitute directly, and find the next term.
Level 2: Routine procedures
Find \(T_n\), \(S_n\), a specific term, or the value of \(n\) using standard methods.
Level 3: Complex procedures
Handle multi-step questions, linked conditions, and convergence questions with a variable.
Level 4: Problem solving
Work in unfamiliar contexts, combine ideas, and interpret patterns in practical situations.
Complete formula reference
| Type | Identify by | General term \(T_n\) | Sum | Notes |
|---|---|---|---|---|
| Arithmetic sequence and arithmetic series | First difference \(d\) is constant | \(a + (n-1)d\) | \(\dfrac{n}{2}[2a + (n-1)d]\) | You can also use \(\frac{n}{2}(a+l)\) when the last term is known. Also remember \(T_n = S_n - S_{n-1}\). |
| Quadratic sequence | Second differences are constant | \(an^2 + bn + c\) | No standard formula | \(a = \frac{\text{second difference}}{2}\), \(3a+b = T_2-T_1\), \(a+b+c = T_1\) |
| Geometric sequence and geometric series | Ratio \(r\) is constant | \(ar^{n-1}\) | \(\dfrac{a(r^n-1)}{r-1}\) | \(\dfrac{a(1-r^n)}{1-r}\) is the equivalent form often used when \(0 < r < 1\). |
| Geometric series to infinity | \(|r| < 1\) | Not required | \(\dfrac{a}{1-r}\) | State the convergence condition before applying the formula. |
Patterns and Sequences usually contributes 25 +/- 3 marks in Paper 1 out of 150 marks. That is roughly 17% of the paper. Both sum formula proofs are directly examinable, and convergence with a variable appears regularly at Level 3 and Level 4.
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Frequently Asked Questions
Short answers to common Grade 12 Patterns, Sequences and Series questions.
How do I identify the type of sequence?
Subtract consecutive terms first. If the first differences are constant, it is an arithmetic sequence. If not, subtract again. If the second differences are constant, it is a quadratic sequence. If dividing consecutive terms gives a constant ratio, it is a geometric sequence.
When does a geometric series converge?
A geometric series converges only when the absolute value of the common ratio is less than 1. In symbols, \(|r| < 1\).
How do I count terms in sigma notation?
Use the rule: number of terms = upper limit - lower limit + 1. For example, \(\sum_{k=3}^{10}\) contains \(10 - 3 + 1 = 8\) terms.
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