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Density of rational numbers · 2 marks

Find 6 rational numbers between 3 and 4.

Answer: 3 = 217 and 4 = 287, so 227, 237, 247, 257, 267, 277 (or 3.1, 3.2, 3.3, 3.4, 3.5, 3.6).

Step-by-step solution

Idea: To get 6 numbers between, write both as fractions with denominator 6 + 1 = 7; the numerators then have exactly 6 whole numbers between them.

  1. Write with denominator 7: 3 = 217 and 4 = 287.1 mark
  2. Numerators 22, 23, 24, 25, 26, 27 lie between 21 and 28, giving 227, 237, 247, 257, 267, 277.1 mark
22/7, 23/7, 24/7, 25/7, 26/7 and 27/7 (other correct answers: 3.1, 3.2, 3.3, 3.4, 3.5, 3.6).

Check: 227 ≈ 3.14 and 277 ≈ 3.86 both lie between 3 and 4 ✓.

Answer to write in the exam

3 = 217, 4 = 287

∴ Six rational numbers: 227, 237, 247, 257, 267, 277

Common mistakes that cost marks

  • Saying there are no numbers between 3 and 4 because they are consecutive integers. There are no integers between them, but infinitely many rational numbers.
  • Including 3 or 4 themselves (e.g. 217 or 287).
  • Using denominator 6, which gives only 5 numbers between (196 to 236).

How this can come in the exam

MCQ (1 mark)

Which of these is NOT between 2 and 3?

  1. 52
  2. 114
  3. 165
  4. 2.01
Show answer

(C) 165
165 = 3.2 > 3.

Short answer (2 marks)

Find four rational numbers between −2 and −1.

Show answer−2 = −105 and −1 = −55 (1 mark). −95, −85, −75, −65 lie between (1 mark).

Try one yourself

Find 4 rational numbers between 5 and 6.

Show answer

5 = 255, 6 = 305: 265, 275, 285, 295.

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