IB Mathematics AHL 5.10 The second derivative-AI HL Paper 1- Exam Style Questions- New Syllabus
The function \( f \) is defined, for \( -\frac{\pi}{2} \leq x \leq \frac{\pi}{2} \), by \( f(x) = 2\cos x + x\sin x \):
(a) Determine whether \( f \) is even, odd, or neither even nor odd [3]
(b) Show that \( f”(0) = 0 \) [2]
(c) John states that, because \( f”(0) = 0 \), the graph of \( f \) has a point of inflection at the point \( (0, 2) \). Explain briefly whether John’s statement is correct or not [2]
▶️ Answer/Explanation
(a)
Compute \( f(-x) \):
\( f(-x) = 2\cos(-x) + (-x)\sin(-x) = 2\cos x + x\sin x = f(x) \)
Since \( f(-x) = f(x) \), the function is even.
Result: \( f \) is even [3]
(b)
First derivative: \( f'(x) = -2\sin x + \sin x + x\cos x = -\sin x + x\cos x \)
Second derivative: \( f”(x) = -\cos x + \cos x – x\sin x = -x\sin x \)
At \( x = 0 \): \( f”(0) = -0 \cdot \sin 0 = 0 \)
Result: \( f”(0) = 0 \) [2]
(c)
Verify point: \( f(0) = 2\cos 0 + 0 \cdot \sin 0 = 2 \), so the point is \( (0, 2) \)
At \( x = 0 \): \( f'(0) = -\sin 0 + 0 \cdot \cos 0 = 0 \), so critical point
Since \( f”(0) = 0 \), compute higher derivatives:
\( f”'(x) = -\sin x – x\cos x \), \( f”'(0) = 0 \)
\( f^{(4)}(x) = -2\cos x + x\sin x \), \( f^{(4)}(0) = -2 < 0 \)
First non-zero derivative is fourth, negative, indicating a maximum.
John’s statement is incorrect; \( (0, 2) \) is a local maximum, not a point of inflection.
Result: John’s statement is incorrect; \( (0, 2) \) is a maximum [2]