IB Mathematics AHL 2.15 Solving inequalities AA HL Paper 3
(a) Sketch the curve \( y = \left| \ln x \right| – \left| \cos x \right| – 0.1 \), \( 0 < x < 4 \), showing clearly the coordinates of the points of intersection with the \( x \)-axis and the coordinates of any local maxima and minima. [5]
(b) Find the values of \( x \) for which \( \left| \ln x \right| > \left| \cos x \right| + 0.1 \), \( 0 < x < 4 \). [2]
▶️ Answer/Explanation
(a) Sketch of \( y = \left| \ln x \right| – \left| \cos x \right| – 0.1 \):
Correct shape, with \( \left| \ln x \right| \) increasing logarithmically and \( \left| \cos x \right| \) oscillating, shifted by \(-0.1\): \( (A1) \)
x-intercepts: Solve \( \left| \ln x \right| = \left| \cos x \right| + 0.1 \) numerically or graphically:
\[ \begin{cases} 0.354 \\ 1.36 \\ 2.59 \\ 2.95 \end{cases} \quad (A2) \]
Note: Award A1 for three correct x-intercepts, A0 otherwise.
Maximum: Compute the derivative (considering cases due to absolute values) to find the critical point at \( x = \frac{\pi}{2} \approx 1.57 \), evaluate \( y \approx 0.352 \):
\[ \left( \frac{\pi}{2}, 0.352 \right) \quad (A1) \]
Minima: Identify critical points or use graphical analysis to find minima:
\[ \begin{cases} (1, -0.640) \\ (2.77, -0.0129) \end{cases} \quad (A1) \]
[5 marks]
(b) Values of \( x \) for which \( \left| \ln x \right| > \left| \cos x \right| + 0.1 \):
Find where \( y = \left| \ln x \right| – \left| \cos x \right| – 0.1 > 0 \), using x-intercepts from part (a) to determine intervals:
\[ \begin{cases} 0 < x < 0.354 \\ 1.36 < x < 2.59 \\ 2.95 < x < 4 \end{cases} \quad (A2) \]
Note: Award A1 if two correct regions are given.
[2 marks]