Try to prove the irrationality of using the approach of proof by contradiction. Will the same approach work for or ?
Step 1: Assumption: Assume is a rational number. It can be written as , in its simplest form, meaning and are co-prime. So,
Squaring both sides gives:
or
Step 2: Deduction for : Because is 3 times an integer, is divisible by 3. If the square of a number is divisible by 3, the number itself must be divisible by 3. Therefore, is divisible by 3. Let . Substituting this gives:
which simplifies to:
Step 3: Contradiction: Because is 3 times an integer, must also be divisible by 3. This contradicts our initial assumption that and share no common factors other than 1. Because our logical steps are flawless, our initial assumption must be wrong.
∴ is irrational.
Step 4: Application to and : Yes, the same proof by contradiction approach works for and . Since 5 and 7 are prime numbers, if is divisible by them, must also be divisible by them, leading to the same contradiction that and are not co-prime.
Step 5: Application to : For , the approach also works. If , is divisible by 10. This implies is divisible by the prime factors of 10 (2 and 5), which again leads to being divisible by those factors, contradicting that and are co-prime.
Explanation
The textbook demonstrates the proof by contradiction for the irrationality of . This question asks students to apply the same logical framework to and then reason about other square roots.
The core mathematical principle required is that if a prime number divides a perfect square, it must also divide the base number. This principle ensures that the assumption of co-prime numerator and denominator always fails, leading to the required contradiction.
Solution Steps
Step 1: Assume (co-prime), square to get .
Step 2: Deduce is divisible by 3, let , substitute to get .
Step 3: Deduce is divisible by 3, contradicting the co-prime assumption, proving is irrational.
Step 4: The approach works for and because they are prime, leading to the same contradiction.
Step 5: The approach works for because divisibility by 10 implies divisibility by its prime factors (2 and 5), leading to the same contradiction.