Question
If \( f(x) = \sec x \) then \( \lim_{x \to \frac{\pi}{3}} \frac{f(x) – f(\frac{\pi}{3})}{x – \frac{\pi}{3}} \) is
A) 0
B) \( \sec \left(\frac{\pi}{3}\right) \)
C) \( \sec \left(\frac{\pi}{3}\right) \tan \left(\frac{\pi}{3}\right) \)
D) nonexistent
▶️ Answer/Explanation
Solution
Correct Answer: C
Concept: The limit expression represents the definition of the derivative of \( \sec x \) at \( x = \frac{\pi}{3} \). Step 1: Recall the derivative of secant: \[ \frac{d}{dx} \sec x = \sec x \tan x \] Step 2: Evaluate at \( x = \frac{\pi}{3} \): \[ f’\left(\frac{\pi}{3}\right) = \sec\left(\frac{\pi}{3}\right) \tan\left(\frac{\pi}{3}\right) \] Numerical values: \[ \sec\left(\frac{\pi}{3}\right) = 2 \] \[ \tan\left(\frac{\pi}{3}\right) = \sqrt{3} \] \[ f’\left(\frac{\pi}{3}\right) = 2\sqrt{3} \]