To determine if the function x³-5x²+3x+5 is injective, one must check for local extrema. A function is not injective if it has a local maximum or minimum, as it will take the same value at different points. Analyzing the derivative can reveal critical points where the function may change direction. If the derivative does not change sign, the function is injective. Thus, examining the behavior of the derivative is essential for establishing injectivity.