To rewrite an expression using the imaginary number \( i \), simply substitute \( i \) wherever you encounter \( \sqrt{-1} \) in the expression. For example, in rewrite expression using imaginary number \( i \) worksheet, if you have \( \sqrt{-9} \), replace it with \( 3i \) because \( \sqrt{-9} = \sqrt{9 \cdot (-1)} = 3i \). This approach allows for handling calculations involving square roots of negative numbers using \( i \), which is essential for solving equations and understanding complex numbers in various theoretical and practical contexts.