Algebra

Linear inequalities

Solve ax + b < c, reverse the inequality when multiplying by a negative, and write the answer in interval notation.

CAPS

Grade 10 · Mathematics · Algebra

This sits on Mathematics Paper 1.

  • The paper

    Paper 1 is algebra, exponents, equations, number patterns, functions, finance and probability. Copy the formula, then substitute.

  • Cognitive level

    CAPS spreads the marks across knowledge, routine procedures, complex procedures and problem solving. A routine question still wants the line of working. A problem-solving question wants you to choose the method, not only to calculate.

  • The relation

    x < a ⇔ x ∈ (−∞; a). Open interval

  • How a marker reads it

    NSC wants a closed circle for ≤ or ≥ and an open circle for < or >.

Each Mathematics paper is 150 marks. A correct substitution can earn a method mark even when the last arithmetic slips, if the formula was the right one.

Diagram

Worked example

Solve −2x + 5 ≥ 11 and represent the solution on a number line.

Simplified approach

Isolate x. Because you divide by −2, flip ≥ to ≤.

Culture note

A taxi fare that cannot go above a Gauteng cap is an inequality — the arrow on the number line is the allowed fares.

Formulas

x < a⇔x ∈ (−∞; a)

Open interval

How to hunt the missing letter

Do not start by naming the letters. First hunt the missing one: copy every number you can see (Given). The letter with no number is Missing. The asking word is Find. The formula that still contains that letter is what you Calculate.

How to hunt: (1) Underline every number and its unit — those are Given. (2) Circle the asking word — find, calculate, how far, what is. That word names Find. (3) The letter the asking word names, which has no number yet, is Missing. (4) Write the formula with empty letters. Cross out every given letter. The letter still standing is what you Calculate.

  • x

    unknown

    How you spot this in the sentence

    When the sentence names the unknown, that letter is x. Copy the number next to it into Given. If the sentence never names it, x is Missing. All numbers that make the inequality true — a set, not one value.

    Example:x < 3 means every number smaller than 3, not just 2.

  • a

    boundary

    How you spot this in the sentence

    When the sentence names the boundary, that letter is a. Copy the number next to it into Given. If the sentence never names it, a is Missing. The number on the right of x < a. Open circle if < or >; closed if ≤ or ≥.

    Example:x < 4 is the interval (−∞; 4).

Exam tricks you must know

These are the moves markers look for — not shortcuts that skip the method, but the habits that earn the method mark.

  • Closed circle for ≤ or ≥, open circle for < or > — it is a separate mark

    How:After you solve, sketch the number line before you write interval notation. Match the circle to the inequality symbol. Then write the interval with [ ] or ( ) to match. Gauteng awards the circle as its own mark.

    When to use it:Inequalities, domains, and “where the graph is above the axis”.

  • “Factorise fully” means HCF and then the identity — leaving 2(x² − 9) loses a mark

    How:Look at the given polynomial. Pull out the biggest number and letter that divides every term (HCF). Then look at what is left: difference of squares, trinomial, or grouping. Both layers go on the page. Stop only when nothing else factorises.

    When to use it:Any “factorise fully” on Paper 1, especially KZN.

  • Memorise exact values for 0°, 30°, 45°, 60°, 90° — compound angles collapse without them

    How:Before a calculator, write the triangle: 30-60-90 and 45-45-90. sin 30° = 12, sin 45° = 1√2, sin 60° = √32, and the cosines swap. Compound-angle questions want these as fractions with division lines, not decimals.

    When to use it:Reduction, compound angles, and “without a calculator” trig.

Step 1 / 5

Subtract 5

−2x ≥ 6.

Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

Solve 2x + 5 > 11.

4. Step-by-step working

  1. 01

    Isolate the unknown

    2x > 6.

    2x > 6
  2. 02

    Divide

    x > 3.

    x > 3

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Solve 2x + 5 > 11.

Before any working — hunt first

Given

Numbers and units already in the sentence: 2x · 5 · 11. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: Solve 2x + 5 > 11. Circle that word — that is the missing variable.

Find

We are required to find: Solve 2x + 5 > 11. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write 2x > 6 with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives Solve 2x + 5 > 11.

In other words — as if you have never seen this before

If you have never seen this before: Treat like an equation, keep the inequality. Divide by a positive number — direction stays. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

2x + 5 > 11, a = 2 > 0 so the sign does not flip.

3. Which method — and why this one?

Treat like an equation, keep the inequality. Divide by a positive number — direction stays.

4. Step-by-step working

  1. 01 / 05

    Isolate the unknown

    2x > 6.

    2x > 6

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  2. 02 / 05

    Divide

    x > 3.

    x > 3

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  3. 03 / 05

    Step G — Sense-check the size

    Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.

    Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.

  4. 04 / 05

    Step F — Box the answer with its unit

    x > 3

    Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.

  5. 05 / 05

    Step H — What the marker is looking for

    On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.

    Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

x > 3

Copy onto your answer sheet — in this order

Formula: 2x > 6

Answer: x > 3

6. Check it like a marker

If you divide by a negative, the inequality flips. Here a is positive.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

Ntsako · fx-991ZA PLUS. Tap to calculate.