Density of Numbers Math Example 1

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Example 1

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Find three rational numbers strictly between 25\dfrac{2}{5} and 35\dfrac{3}{5}.

Solution

  1. 1
    Method 1 (mediant / averaging): Average of 25\dfrac{2}{5} and 35\dfrac{3}{5}: 12(25+35)=12=0.5\dfrac{1}{2}\left(\dfrac{2}{5}+\dfrac{3}{5}\right) = \dfrac{1}{2} = 0.5. First number: 12\dfrac{1}{2}.
  2. 2
    Average of 25\dfrac{2}{5} and 12\dfrac{1}{2}: 12(410+510)=920\dfrac{1}{2}\left(\dfrac{4}{10}+\dfrac{5}{10}\right) = \dfrac{9}{20}. Second number: 920\dfrac{9}{20}.
  3. 3
    Average of 12\dfrac{1}{2} and 35\dfrac{3}{5}: 12(510+610)=1120\dfrac{1}{2}\left(\dfrac{5}{10}+\dfrac{6}{10}\right) = \dfrac{11}{20}. Third number: 1120\dfrac{11}{20}.
  4. 4
    Check order: 25=820<920<1020<1120<1220=35\dfrac{2}{5} = \dfrac{8}{20} < \dfrac{9}{20} < \dfrac{10}{20} < \dfrac{11}{20} < \dfrac{12}{20} = \dfrac{3}{5}. βœ“

Answer

Three rationals between 25\dfrac{2}{5} and 35\dfrac{3}{5}: 920\dfrac{9}{20}, 12\dfrac{1}{2}, 1120\dfrac{11}{20}.
The rationals are dense: between any two distinct rationals there are infinitely many more. Repeated averaging is a simple constructive method. This property β€” density β€” distinguishes rationals and reals from integers.

About Density of Numbers

The property that between any two distinct real numbers, there are infinitely many other real numbersβ€”no two are 'adjacent'.

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