What is the (mn)-term in ( (2m+n-1)^2 )?

Correct answer: \(4mn\)

In \((2m+n-1)^2\), the \(mn\)-term is formed only by multiplying \(2m\) and \(n\). In the expansion of a square, this cross-term is \(2\times(2m)\times n=4mn\). Therefore, the correct answer is \(4mn\). It cannot be \(-4mn\) because both \(2m\) and \(n\) are positive terms. Exam tip: for a particular mixed term in the square of three terms, use \(2ab\) only for the relevant pair.