Proof of irrationality of √2, √3, √5

Correct answer: A. Square both sides

Explanation: Step 1: In the proof, we assume (\sqrt{2}=\frac{a}{b}). Step 2: To remove the square root, we square both sides. Step 3: In exams, do not skip the squaring step.

Correct answer: A. (b\neq 0)

Explanation: Step 1: The denominator of a fraction cannot be zero. Step 2: So while writing (\frac{a}{b}), the condition (b\neq 0) is necessary. Step 3: Write this condition when expressing a rational number.

Correct answer: A. Because the fraction is taken in lowest form

Explanation: Step 1: A rational number is written as a ratio of two integers. Step 2: In the proof, it is taken in lowest form, so (m) and (n) are coprime. Step 3: Later, finding a common factor gives the contradiction.

Correct answer: A. (2)

Explanation: Step 1: The right side of the equation is (2b^2). Step 2: So (a^2) has factor (2) and is divisible by (2). Step 3: Use the factor to decide divisibility.

Correct answer: A. (a) is divisible by (3)

Explanation: Step 1: From (a^2=3b^2), (a^2) is divisible by (3). Step 2: Since (3) is prime, (a) is also divisible by (3). Step 3: Remember this rule from square to original number.

Correct answer: A. (a=5k), where (k) is an integer

Explanation: Step 1: From (a^2=5b^2), (a^2) is divisible by (5). Step 2: Therefore (a) is also divisible by (5), so (a=5k). Step 3: After divisibility, write the number using that factor.

Correct answer: A. (4k^2)

Explanation: Step 1: We have (a=2k). Step 2: Squaring gives (a^2=(2k)^2=4k^2). Step 3: Do not forget to square the coefficient also.

Correct answer: A. (9k^2)

Explanation: Step 1: (a=3k). Step 2: Squaring gives ((3k)^2=9k^2). Step 3: Write this algebraic step carefully in the proof of (\sqrt{3}).

Correct answer: A. (25k^2)

Explanation: Step 1: We write (a=5k). Step 2: Squaring gives (a^2=(5k)^2=25k^2). Step 3: A small algebra mistake can spoil the whole proof.

Correct answer: A. Because then both have (2) as a common factor

Explanation: Step 1: An even number is divisible by (2). Step 2: If both (a) and (b) are even, both have (2) as a common factor. Step 3: This contradicts the coprime condition.

Correct answer: A. The condition of being coprime

Explanation: Step 1: Coprime numbers have no common factor except (1). Step 2: If both (a) and (b) are divisible by (3), they have common factor (3). Step 3: This is the contradiction in the proof.

Correct answer: A. They cannot be coprime

Explanation: Step 1: If both are divisible by (5), both have (5) as a common factor. Step 2: This cannot happen for coprime numbers. Step 3: Thus the initial rational assumption becomes false.

Correct answer: A. Assume rational, square, get contradiction through evenness

Explanation: Step 1: First assume (\sqrt{2}) is rational. Step 2: Then square and use (a^2=2b^2) to get evenness results. Step 3: Finally, the coprime condition gives a contradiction.

Correct answer: A. (a=3b)

Explanation: Step 1: From (a^2=3b^2), (a^2) is divisible by (3). Step 2: Then (a) is divisible by (3), so (a=3k). Step 3: Directly writing (a=3b) from the equation is wrong.

Correct answer: A. (a^2) is divisible by (5)

Explanation: Step 1: The right side of the equation is (5b^2). Step 2: Therefore the left side (a^2) is also divisible by (5). Step 3: First write divisibility of the square, then of the original number.

Correct answer: A. We assume the opposite of what is to be proved and show an impossible result

Explanation: Step 1: In contradiction, we take the opposite assumption. Step 2: If it leads to an impossible result, the original statement is proved true. Step 3: This method is very useful in irrationality proofs.

Correct answer: A. Integer

Explanation: Step 1: An even integer is written as (2) times an integer. Step 2: So in (a=2k), (k) is an integer. Step 3: It is good to mention the type of (k) in such forms.

Correct answer: A. (a) is divisible by (3)

Explanation: Step 1: From (a^2=3b^2), (a) is found divisible by (3). Step 2: A number divisible by (3) is written as (3k). Step 3: This form helps show divisibility of (b) later.

Correct answer: A. To show that (b) is also divisible by (5)

Explanation: Step 1: First, (a) is found divisible by (5). Step 2: Substituting (a=5k) gives divisibility by (5) for (b) too. Step 3: Getting a common factor in both is the contradiction.

Correct answer: A. (a) and (b) were assumed coprime, but both turned out even

Explanation: Step 1: At the beginning, (\frac{a}{b}) is taken in lowest form. Step 2: The proof shows both (a) and (b) are even. Step 3: Being coprime and both even is impossible.

Correct answer: A. Because (2), (3), and (5) are not perfect squares

Explanation: Step 1: Square roots of perfect squares are integers. Step 2: (2), (3), and (5) are not perfect squares. Step 3: That is why irrationality proofs are studied for their square roots.

Correct answer: A. (\sqrt{9})

Explanation: Step 1: (9) is a perfect square. Step 2: (\sqrt{9}=3), which is rational. Step 3: A square root of a perfect square does not need an irrationality proof.

Correct answer: A. As a ratio of two integers

Explanation: Step 1: A rational number can be written as a ratio of two integers. Step 2: So after assuming rationality, we write (\sqrt{2}=\frac{a}{b}). Step 3: The definition of rationality starts the proof.

Correct answer: A. (5)

Explanation: Step 1: The square of (\sqrt{5}) is (5). Step 2: So after squaring both sides, the left side becomes (5). Step 3: A square root and square cancel each other.

Correct answer: A. \(\frac{a^2}{b^2}\)

Explanation: Step 1: While squaring a fraction, both numerator and denominator are squared. Step 2: Therefore \(\left(\frac{a}{b}\right)^2=\frac{a^2}{b^2}\). Step 3: Squaring only the numerator is a mistake.