Study Guides/Maths/Every Irrational Number is a Real Number
Study Guide · Maths

Is Every Irrational Number a Real Number?

This statement is a classic 'True or False' question found in the very first exercise of the Class 9 NCERT Mathematics textbook (Number Systems). To answer it, we must understand the family tree of numbers.

Question (Click to Flip)

What is an imaginary number?

Answer

If a number is NOT a real number, it is an 'Imaginary' or 'Complex' number. This happens when you try to find the square root of a negative number (like $\sqrt{-5}$), which cannot exist on a standard number line.

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Key Facts

The combination of all Rational numbers and all Irrational numbers put together forms the collection of Real numbers.

The opposite statement—'Every real number is an irrational number'—is False, because a real number could also be a rational number like 4.

1. The Answer

Statement: Every irrational number is a real number. Answer: True.

2. Mathematical Reason (The Family Tree)

  • The Real Number System (denoted by $\mathbb{R}$) is the grand 'father' category of all numbers that can be plotted on a physical number line.
  • The Real Number System is strictly divided into exactly two child categories:
    1. Rational Numbers: Numbers that can be written as a simple fraction $p/q$ (e.g., $1/2, 5, -3, 0.75$).
    2. Irrational Numbers: Numbers that CANNOT be written as a fraction, and have endless, non-repeating decimals (e.g., $\sqrt{2}, \pi, \sqrt{3}$).
  • Because Irrational Numbers are born directly from the Real Number family, every single irrational number is, by definition, a part of the Real Number collection.

3. The Number Line Proof

A Real Number is defined as any number that has a specific, unique point on the number line. Even though irrational numbers like $\sqrt{2}$ (1.4142...) go on forever, we can geometrically plot their exact location on a number line using the Pythagoras theorem. Since it exists on the line, it is a real number.

Questions and Answers

What is an imaginary number?+

If a number is NOT a real number, it is an 'Imaginary' or 'Complex' number. This happens when you try to find the square root of a negative number (like $\sqrt{-5}$), which cannot exist on a standard number line.

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