\( \lim _ { + \infty } \frac { \sqrt { x - 1 } - 3 } { x + 3 } \)
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To evaluate the limit \( \lim _ { + \infty } \frac { \sqrt { x - 1 } - 3 } { x + 3 } \), we start by manipulating the expression in a way that simplifies it as \( x \) approaches infinity. First, we can rewrite the square root term: \[ \sqrt{x - 1} = \sqrt{x(1 - \frac{1}{x})} = \sqrt{x} \sqrt{1 - \frac{1}{x}}. \] As \( x \) approaches infinity, \( \sqrt{1 - \frac{1}{x}} \) approaches 1, so we can approximate \( \sqrt{x - 1} \approx \sqrt{x} \) at large values of \( x \). Then, our limit becomes: \[ \lim_{x \to +\infty} \frac{\sqrt{x} - 3}{x + 3}. \] To further simplify, we divide the numerator and the denominator by \( \sqrt{x} \): \[ \lim_{x \to +\infty} \frac{1 - \frac{3}{\sqrt{x}}}{\frac{x}{\sqrt{x}} + \frac{3}{\sqrt{x}}} = \lim_{x \to +\infty} \frac{1 - \frac{3}{\sqrt{x}}}{\sqrt{x} + \frac{3}{\sqrt{x}}}. \] As \( x \) approaches infinity, \( \frac{3}{\sqrt{x}} \) approaches 0. Therefore, the limit simplifies to: \[ \lim_{x \to +\infty} \frac{1 - 0}{\sqrt{x} + 0} = \lim_{x \to +\infty} \frac{1}{\sqrt{x}} = 0. \] Thus, the final result is: \[ \boxed{0}. \]