Question 30 of 58advanced🔧 ApplyLong Answer5 marks

Try to prove the irrationality of 3\sqrt{3} using the approach of proof by contradiction. Will the same approach work for 5,7,\sqrt{5}, \sqrt{7}, or 10\sqrt{10}?

Correct Answer

Step 1: Assumption: Assume 3\sqrt{3} is a rational number. It can be written as pq\frac{p}{q}, q≠0q \neq 0 in its simplest form, meaning pp and qq are co-prime. So,

3=pq\sqrt{3} = \frac{p}{q}

Squaring both sides gives:

3=p2q23 = \frac{p^2}{q^2}

or

3q2=p23q^2 = p^2

Step 2: Deduction for pp: Because p2p^2 is 3 times an integer, p2p^2 is divisible by 3. If the square of a number is divisible by 3, the number itself must be divisible by 3. Therefore, pp is divisible by 3. Let p=3kp = 3k. Substituting this gives:

3q2=(3k)2=9k23q^2 = (3k)^2 = 9k^2

which simplifies to:

q2=3k2q^2 = 3k^2

Step 3: Contradiction: Because q2q^2 is 3 times an integer, qq must also be divisible by 3. This contradicts our initial assumption that pp and qq share no common factors other than 1. Because our logical steps are flawless, our initial assumption must be wrong.

∴ 3\sqrt{3} is irrational.

Step 4: Application to 5\sqrt{5} and 7\sqrt{7}: Yes, the same proof by contradiction approach works for 5\sqrt{5} and 7\sqrt{7}. Since 5 and 7 are prime numbers, if p2p^2 is divisible by them, pp must also be divisible by them, leading to the same contradiction that pp and qq are not co-prime.

Step 5: Application to 10\sqrt{10}: For 10\sqrt{10}, the approach also works. If 10q2=p210q^2 = p^2, p2p^2 is divisible by 10. This implies pp is divisible by the prime factors of 10 (2 and 5), which again leads to qq being divisible by those factors, contradicting that pp and qq are co-prime.

Exercise: Think and Reflect | Q: 1 | (Page 15)
For More Understanding

Explanation

The textbook demonstrates the proof by contradiction for the irrationality of 2\sqrt{2}. This question asks students to apply the same logical framework to 3\sqrt{3} 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

  1. Step 1: Assume 3=pq\sqrt{3} = \frac{p}{q} (co-prime), square to get 3q2=p23q^2 = p^2.

  2. Step 2: Deduce pp is divisible by 3, let p=3kp=3k, substitute to get q2=3k2q^2 = 3k^2.

  3. Step 3: Deduce qq is divisible by 3, contradicting the co-prime assumption, proving 3\sqrt{3} is irrational.

  4. Step 4: The approach works for 5\sqrt{5} and 7\sqrt{7} because they are prime, leading to the same contradiction.

  5. Step 5: The approach works for 10\sqrt{10} because divisibility by 10 implies divisibility by its prime factors (2 and 5), leading to the same contradiction.