As a mathematician with a deep understanding of number theory, I can provide a comprehensive explanation on the nature of the product of a rational and an irrational number. Let's delve into the definitions and properties of these numbers to better understand the outcome of their multiplication.

Rational Numbers are numbers that can be expressed as a fraction \( \frac{a}{b} \), where \( a \) and \( b \) are integers, and \( b \neq 0 \). This includes all integers and fractions. Rational numbers can be finite decimals or infinite repeating decimals.

Irrational Numbers, on the other hand, are numbers that cannot be expressed as a fraction of two integers. They are non-repeating, non-terminating decimals. Examples of irrational numbers include the square root of a non-perfect square (like \( \sqrt{2} \) or \( \sqrt{3} \)), \( \pi \) (pi), and the base of the natural logarithm, \( e \).

Now, let's consider the product of a non-zero rational number and an irrational number. The statement we are examining is:

> **The product of a non-zero rational number and an irrational number is irrational.**

To prove this statement, we can use an indirect proof, also known as proof by contradiction. This method involves assuming the opposite of what we want to prove and then showing that this assumption leads to a contradiction of a known fact.

Indirect Proof:


1. Assumption for Contradiction: Let's assume that the product of a non-zero rational number \( r \) and an irrational number \( i \) is not irrational. This means that their product \( r \cdot i \) is a rational number.


2. Contradiction from Rationality: If \( r \cdot i \) is rational, then it can be expressed as a fraction \( \frac{a}{b} \), where \( a \) and \( b \) are integers and \( b \neq 0 \).


3. Distribution of Multiplication: By the distributive property of multiplication over addition, we can express \( r \) as \( \frac{a}{b} \) multiplied by \( i \), which gives us \( \frac{a}{b} \cdot i \).


4. Rationalizing the Expression: Since \( i \) is irrational, we cannot simplify \( \frac{a}{b} \cdot i \) to a fraction of two integers. However, our initial assumption was that \( r \cdot i \) is rational, which contradicts the nature of irrational numbers.


5. Reaching a Contradiction: The contradiction arises because we have assumed that a product involving an irrational number can be rational, which is not possible. This is because the property of irrational numbers dictates that their decimal expansion is non-repeating and non-terminating, and there is no way to pair this with a rational number to result in another rational number through multiplication.


6. Conclusion of the Proof: Since our assumption leads to a contradiction, we must reject it. Therefore, the original statement stands: the product of a non-zero rational number and an irrational number is indeed irrational.

This proof relies on the fundamental properties of rational and irrational numbers and the rules of arithmetic. It is a classic example of how mathematical proofs can be constructed to establish the validity of a statement through contradiction.

Now, let's translate this explanation into Chinese.

read more >>

"The product of a non-zero rational number and an irrational number is irrational." Indirect Proof (Proof by Contradiction) of the better statement: (Assume the opposite of what you want to prove, and show it leads to a contradiction of a known fact.)read more >>