Shortcut Trick
The difference between two distinct irrational numbers (√2 and √3) where neither is a multiple of the other by a rational factor always results in an irrational number.
√2 ≈ 1.4142... and √3 ≈ 1.7320...
√2 − √3 ≈ −0.3178... which is non-terminating and non-repeating.
Such decimals represent irrational numbers.
∴ The correct answer is an irrational number.
Alternate Method
Given: The expression is √2 − √3.
Method: Proof by Contradiction.
⇒ Let us assume that √2 − √3 = r, where r is a rational number.
⇒ Squaring both sides: (√2 − √3)2 = r2
⇒ (√2)2 + (√3)2 − 2(√2 × √3) = r2
⇒ 2 + 3 − 2√6 = r2
⇒ 5 − 2√6 = r2
⇒ 5 − r2 = 2√6
⇒ √6 = (5 − r2) / 2
⇒ Since r is rational, r2 is rational, and (5 − r2) / 2 is also rational.
⇒ However, √6 is an irrational number (as 6 is not a perfect square).
⇒ An irrational number cannot be equal to a rational number.
⇒ This is a contradiction, so our assumption was wrong.
∴ The correct answer is an irrational number.
Additional Information
Irrational Numbers
These are numbers that cannot be expressed as a ratio of two integers (p/q). Their decimal expansion is non-terminating and non-repeating.
Properties of Irrationals
The sum or difference of a rational and an irrational number is always irrational. The sum or difference of two irrationals may be rational or irrational.
Square Roots of Primes
The square root of any prime number (like 2, 3, 5, 7, etc.) is always an irrational number.
Real Numbers
Real numbers are the union of rational numbers and irrational numbers. Every point on the number line represents a unique real number.