Which of the following algebraic identities can be used to factorise the expression \(x^2-9\)?

Correct answer: \(a^2-b^2=(a-b)(a+b)\)

The expression \(x^2-9\) can be written as \(x^2-3^2\), which is a difference of two squares. Therefore, using \(a^2-b^2=(a-b)(a+b)\), we get \(x^2-9=(x-3)(x+3)\). Option B is incorrect because the identity for the square of a difference contains the middle term \(-2ab\). Exam tip: when two perfect squares are separated by subtraction, check for the difference-of-squares identity first.