Home / IBDP Maths SL 5.6 Derivatives of xn , sinx , cosx , tanx AA HL Paper 1- Exam Style Questions

IBDP Maths SL 5.6 Derivatives of xn , sinx , cosx , tanx AA HL Paper 1- Exam Style Questions

IBDP Maths SL 5.6 Derivatives of xn , sinx , cosx , tanx and lnx AA HL Paper 1- Exam Style Questions- New Syllabus

Question

Consider the function \(f(x) = \frac{\ln x}{x}\), \(0 < x < e^2\).

a. (i) Solve the equation \(f'(x) = 0\).
(ii) Hence show the graph of \(f\) has a local maximum.
(iii) Write down the range of the function \(f\). [5]

b. Show that there is a point of inflexion on the graph and determine its coordinates. [5]

c. Sketch the graph of \(y = f(x)\), indicating clearly the asymptote, x-intercept and the local maximum. [3]

d. Now consider the functions \(g(x) = \frac{\ln|x|}{x}\) and \(h(x) = \frac{\ln|x|}{|x|}\), where \(0 < x < e^2\).
(i) Sketch the graph of \(y = g(x)\).
(ii) Write down the range of \(g\).
(iii) Find the values of \(x\) such that \(h(x) > g(x)\). [6]

▶️ Answer/Explanation
Solution a

(i) \(f'(x) = \frac{x\frac{1}{x} – \ln x}{x^2}\) M1A1

\( = \frac{1 – \ln x}{x^2}\)

so \(f'(x) = 0\) when \(\ln x = 1\), i.e. \(x = e\) A1

(ii) \(f'(x) > 0\) when \(x < e\) and \(f'(x) < 0\) when \(x > e\) R1

hence local maximum AG

Note: Accept argument using correct second derivative.

(iii) \(y \leq \frac{1}{e}\) A1

[5 marks]

Solution b

\(f”(x) = \frac{x^2\frac{-1}{x} – (1 – \ln x)2x}{x^4}\) M1

\( = \frac{-x – 2x + 2x\ln x}{x^4}\)

\( = \frac{-3 + 2\ln x}{x^3}\) A1

Note: May be seen in part (a).

\(f”(x) = 0\) (M1)

\(-3 + 2\ln x = 0\)

\(x = e^{\frac{3}{2}}\)

since \(f”(x) < 0\) when \(x < e^{\frac{3}{2}}\) and \(f”(x) > 0\) when \(x > e^{\frac{3}{2}}\) R1

then point of inflexion \(\left(e^{\frac{3}{2}}, \frac{3}{2e^{\frac{3}{2}}}\right)\) A1

[5 marks]

Solution c
Graph of f(x)

A1A1A1

Note: Award A1 for the maximum and intercept, A1 for a vertical asymptote and A1 for shape (including turning concave up).

[3 marks]

Solution d

(i)

Graph of g(x)

A1A1

Note: Award A1 for each correct branch.

(ii) all real values A1

(iii)

Graph comparison

(M1)(A1)

Note: Award (M1)(A1) for sketching the graph of h, ignoring any graph of g.

\(-e^2 < x < -1\) (accept \(x < -1\)) A1

[6 marks]

Question

The diagram below shows a circular lake with centre O, diameter AB and radius 2 km.

Circular lake diagram

Jorg needs to get from A to B as quickly as possible. He considers rowing to point P and then walking to point B. He can row at \(3{\text{ km}}\,{{\text{h}}^{ – 1}}\) and walk at \(6{\text{ km}}\,{{\text{h}}^{ – 1}}\). Let \({\rm{P\hat AB}} = \theta \) radians, and t be the time in hours taken by Jorg to travel from A to B.

a. Show that \(t = \frac{2}{3}(2\cos \theta + \theta )\). [3]

b. Find the value of \(\theta \) for which \(\frac{{{\text{d}}t}}{{{\text{d}}\theta }} = 0\). [2]

c. What route should Jorg take to travel from A to B in the least amount of time?

Give reasons for your answer. [3]

▶️ Answer/Explanation
Solution a

angle APB is a right angle

\(\Rightarrow \cos \theta = \frac{{\text{AP}}}{4} \Rightarrow {\text{AP}} = 4\cos \theta \) A1

Note: Allow correct use of cosine rule.

\({\text{arc PB}} = 2 \times 2\theta = 4\theta \) A1

\(t = \frac{{\text{AP}}}{3} + \frac{{\text{PB}}}{6}\) M1

Note: Allow use of their AP and their PB for the M1.

\(\Rightarrow t = \frac{4\cos \theta}{3} + \frac{4\theta}{6} = \frac{4\cos \theta}{3} + \frac{2\theta}{3} = \frac{2}{3}(2\cos \theta + \theta)\) AG

[3 marks]

Solution b

\(\frac{{\text{d}}t}{{\text{d}}\theta} = \frac{2}{3}(-2\sin \theta + 1)\) A1

\(\frac{2}{3}(-2\sin \theta + 1) = 0 \Rightarrow \sin \theta = \frac{1}{2} \Rightarrow \theta = \frac{\pi}{6}\) (or 30 degrees) A1

[2 marks]

Solution c

\(\frac{{\text{d}^2}t}{{\text{d}\theta^2}} = -\frac{4}{3}\cos \theta < 0\,\,\,\,\left(\text{at }\theta = \frac{\pi}{6}\right)\) M1

\(\Rightarrow t\) is maximized at \(\theta = \frac{\pi}{6}\) R1

time needed to walk along arc AB is \(\frac{2\pi}{6}\) (≈ 1 hour)

time needed to row from A to B is \(\frac{4}{3}\) (≈ 1.33 hour)

hence, time is minimized in walking from A to B R1

[3 marks]

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