4. \( r^{2}-9 t^{2} \)

Did you know that the expression \( r^{2}-9 t^{2} \) is a classic example of a difference of squares? This algebraic identity can be factored into \( (r - 3t)(r + 3t) \). It’s a handy technique used in algebra to simplify expressions and solve equations efficiently, making it one of the foundational tools in mathematics! In practical terms, difference of squares pops up in various real-world scenarios, such as physics and engineering. For instance, when calculating the difference in squared velocities or energy levels in quantum mechanics, the concept provides a shortcut to work through complex problems. So next time you encounter such terms, think of the clever math tricks hiding behind them!