IBDP Maths AI: Topic: SL 2.2: Concept of a function: IB style Questions SL Paper2

Question

The diagram shows a sketch of the function f (x) = 4x3 − 9x2 − 12x + 3.

Write down the values of x where the graph of f (x) intersects the x-axis.[3]

a.

Write down f ′(x).[3]

b.

Find the value of the local maximum of y = f (x).[4]

c.

Let P be the point where the graph of f (x) intersects the y axis.

Write down the coordinates of P.[1]

d.

Let P be the point where the graph of f (x) intersects the y axis.

Find the gradient of the curve at P.[2]

e.

The line, L, is the tangent to the graph of f (x) at P.

Find the equation of L in the form y = mx + c.[2]

f.

There is a second point, Q, on the curve at which the tangent to f (x) is parallel to L.

Write down the gradient of the tangent at Q.[1]

g.

There is a second point, Q, on the curve at which the tangent to f (x) is parallel to L.

Calculate the x-coordinate of Q.[3]

h.
Answer/Explanation

Markscheme

–1.10, 0.218, 3.13     (A1)(A1)(A1)[3 marks]

a.

f ′(x) = 12x2 – 18x – 12     (A1)(A1)(A1)

Note: Award (A1) for each correct term and award maximum of (A1)(A1) if other terms seen.[3 marks]

b.

′(x) = 0     (M1)
x = –0.5, 2
x = –0.5     (A1)

Note: If x = –0.5 not stated, can be inferred from working below.

y = 4(–0.5)3 – 9(–0.5)2 – 12(–0.5) + 3     (M1)
y = 6.25     (A1)(G3)

Note: Award (M1) for their value of x substituted into f (x).

Award (M1)(G2) if sketch shown as method. If coordinate pair given then award (M1)(A1)(M1)(A0). If coordinate pair given with no working award (G2).[4 marks]

c.

(0, 3)     (A1)

Note: Accept x = 0, y = 3.[1 mark]

d.

′(0) = –12     (M1)(A1)(ft)(G2)

Note: Award (M1) for substituting x = 0 into their derivative.[2 marks]

e.

Tangent: y = –12x + 3     (A1)(ft)(A1)(G2)

Note: Award (A1)(ft) for their gradient, (A1) for intercept = 3.

Award (A1)(A0) if y = not seen.[2 marks]

f.

–12     (A1)(ft)

Note: Follow through from their part (e).[1 mark]

g.

12x2 – 18x – 12 = –12     (M1)

12x2 – 18x = 0     (M1)

x = 1.5, 0

At Q x = 1.5     (A1)(ft)(G2)

Note: Award (M1)(G2) for 12x2 – 18x – 12 = –12 followed by x = 1.5.

Follow through from their part (g).[3 marks]

h.

Question

Consider the function \(f(x) = {x^3} + \frac{{48}}{x}{\text{, }}x \ne 0\).

Calculate \(f(2)\) .[2]

a.

Sketch the graph of the function \(y = f(x)\) for \( – 5 \leqslant x \leqslant 5\) and \( – 200 \leqslant y \leqslant 200\) .[4]

b.

Find \(f'(x)\) .[3]

c.

Find \(f'(2)\) .[2]

d.

Write down the coordinates of the local maximum point on the graph of \(f\) .[2]

e.
Find the range of \(f\) .[3]
f.

Find the gradient of the tangent to the graph of \(f\) at \(x = 1\).[2]

g.

There is a second point on the graph of \(f\) at which the tangent is parallel to the tangent at \(x = 1\).

Find the \(x\)-coordinate of this point.[2]

h.
Answer/Explanation

Markscheme

\(f(2) = {2^3} + \frac{{48}}{2}\)     (M1)

\(= 32\)     (A1)(G2)[2 marks]

a.

(A1) for labels and some indication of scale in an appropriate window
(A1) for correct shape of the two unconnected and smooth branches
(A1) for maximum and minimum in approximately correct positions
(A1) for asymptotic behaviour at \(y\)-axis     (A4)

Notes: Please be rigorous.
The axes need not be drawn with a ruler.
The branches must be smooth: a single continuous line that does not deviate from its proper direction.
The position of the maximum and minimum points must be symmetrical about the origin.
The \(y\)-axis must be an asymptote for both branches. Neither branch should touch the axis nor must the curve approach the
asymptote then deviate away later.[4 marks]

b.

\(f'(x) = 3{x^2} – \frac{{48}}{{{x^2}}}\)     (A1)(A1)(A1)

Notes: Award (A1) for \(3{x^2}\) , (A1) for \( – 48\) , (A1) for \({x^{ – 2}}\) . Award a maximum of (A1)(A1)(A0) if extra terms seen.[3 marks]

c.

\(f'(2) = 3{(2)^2} – \frac{{48}}{{{{(2)}^2}}}\)     (M1)

Note: Award (M1) for substitution of \(x = 2\) into their derivative.

\(= 0\)     (A1)(ft)(G1)[2 marks]

d.

\(( – 2{\text{, }} – 32)\) or \(x = – 2\), \(y = – 32\)     (G1)(G1)

Notes: Award (G0)(G0) for \(x = – 32\), \(y = – 2\) . Award at most (G0)(G1) if parentheses are omitted.[2 marks]

e.

\(\{ y \geqslant 32\}  \cup \{ y \leqslant – 32\} \)     (A1)(A1)(ft)(A1)(ft)

Notes: Award (A1)(ft) \(y \geqslant 32\) or \(y > 32\) seen, (A1)(ft) for \(y \leqslant – 32\) or \(y < – 32\) , (A1) for weak (non-strict) inequalities used in both of the above.
Accept use of \(f\) in place of \(y\). Accept alternative interval notation.
Follow through from their (a) and (e).
If domain is given award (A0)(A0)(A0).
Award (A0)(A1)(ft)(A1)(ft) for \([ – 200{\text{, }} – 32]\) , \([32{\text{, }}200]\).
Award (A0)(A1)(ft)(A1)(ft) for \(] – 200{\text{, }} – 32]\) , \([32{\text{, }}200[\).[3 marks]

f.

\(f'(1) = – 45\)     (M1)(A1)(ft)(G2)

Notes: Award (M1) for \(f'(1)\) seen or substitution of \(x = 1\) into their derivative. Follow through from their derivative if working is seen.[2 marks]

g.

\(x = – 1\)     (M1)(A1)(ft)(G2)

Notes: Award (M1) for equating their derivative to their \( – 45\) or for seeing parallel lines on their graph in the approximately correct position.[2 marks]

h.

Question

The function \(f(x)\) is defined by \(f(x) = 1.5x + 4 + \frac{6}{x}{\text{, }}x \ne 0\) .

Write down the equation of the vertical asymptote.[2]

a.

Find \(f'(x)\) .[3]

b.

Find the gradient of the graph of the function at \(x = – 1\).[2]

c.

Using your answer to part (c), decide whether the function \(f(x)\) is increasing or decreasing at \(x = – 1\). Justify your answer.[2]

d.

Sketch the graph of \(f(x)\) for \( – 10 \leqslant x \leqslant 10\) and \( – 20 \leqslant y \leqslant 20\) .[4]

e.

\({{\text{P}}_1}\) is the local maximum point and \({{\text{P}}_2}\) is the local minimum point on the graph of \(f(x)\) .

Using your graphic display calculator, write down the coordinates of

(i)     \({{\text{P}}_1}\) ;

(ii)    \({{\text{P}}_2}\) .[4]

f.

Using your sketch from (e), determine the range of the function \(f(x)\) for \( – 10 \leqslant x \leqslant 10\) .[3]

g.
Answer/Explanation

Markscheme

\(x = 0\)     (A1)(A1)

Note: Award (A1) for \(x = {\text{constant}}\), (A1) for \(0\).[2 marks]

a.

\(f'(x) = 1.5 – \frac{6}{{{x^2}}}\)     (A1)(A1)(A1)

Notes: Award (A1) for \(1.5\), (A1) for \( – 6\), (A1) for \({x^{ – 2}}\) . Award (A1)(A1)(A0) at most if any other term present.[3 marks]

b.

\(1.5 – \frac{6}{{( – 1)}}\)     (M1)

\( =  – 4.5\)     (A1)(ft)(G2)

Note: Follow through from their derivative function.[2 marks]

c.

Decreasing, the derivative (gradient or slope) is negative (at \(x = – 1\))     (A1)(R1)(ft)

Notes: Do not award (A1)(R0). Follow through from their answer to part (c).[2 marks]

d.

     (A4)
Notes: Award (A1) for labels and some indication of scales and an appropriate window.

Award (A1) for correct shape of the two unconnected, and smooth branches.

Award (A1) for the maximum and minimum points in the approximately correct positions.

Award (A1) for correct asymptotic behaviour at \(x = 0\) .

Notes: Please be rigorous.

The axes need not be drawn with a ruler.

The branches must be smooth and single continuous lines that do not deviate from their proper direction.

The max and min points must be symmetrical about point \((0{\text{, }}4)\) .

The \(y\)-axis must be an asymptote for both branches.[4 marks]

e.

(i)     \(( – 2{\text{, }} – 2)\) or \(x = – 2\), \(y = – 2\)     (G1)(G1)

(ii)    \((2{\text{, }}10)\) or \(x = 2\), \(y = 10\)     (G1)(G1)[4 marks]

f.

\(\{  – 2 \geqslant y\} \) or \(\{ y \geqslant 10\} \)     (A1)(A1)(ft)(A1)

Notes: (A1)(ft) for \(y > 10\) or \(y \geqslant 10\) . (A1)(ft) for \(y < – 2\) or \(y \leqslant – 2\) . (A1) for weak (non-strict) inequalities used in both of the above. Follow through from their (e) and (f).[3 marks]

g.

Question

Consider the function \(f(x) =  – \frac{1}{3}{x^3} + \frac{5}{3}{x^2} – x – 3\).

Sketch the graph of y = f (x) for −3 ≤ x ≤ 6 and −10 ≤ y ≤ 10 showing clearly the axes intercepts and local maximum and minimum points. Use a scale of 2 cm to represent 1 unit on the x-axis, and a scale of 1 cm to represent 1 unit on the y-axis.[4]

a.

Find the value of f (−1).[2]

b.

Write down the coordinates of the y-intercept of the graph of f (x).[1]

c.

Find f ‘(x).[3]

d.

Show that \(f'( – 1) =  – \frac{{16}}{3}\).[1]

e.

Explain what f (−1) represents.[2]

f.

Find the equation of the tangent to the graph of f (x) at the point where x is –1.[2]

g.

Sketch the tangent to the graph of f (x) at x = −1 on your diagram for (a).[2]

h.

P and Q are points on the curve such that the tangents to the curve at these points are horizontal. The x-coordinate of P is a, and the x-coordinate of Q is b, b > a.

Write down the value of

(i) a ;

(ii) b .[2]

i.

P and Q are points on the curve such that the tangents to the curve at these points are horizontal. The x-coordinate of P is a, and the x-coordinate of Q is b, b > a.

Describe the behaviour of f (x) for a < x < b.[1]

j.
Answer/Explanation

Markscheme

(A1) for indication of window and labels. (A1) for smooth curve that does not enter the first quadrant, the curve must consist of one line only.

(A1) for x and y intercepts in approximately correct positions (allow ±0.5).

(A1) for local maximum and minimum in approximately correct position. (minimum should be 0 ≤ x ≤ 1 and –2 ≤ y ≤ –4 ), the y-coordinate of the maximum should be 0 ± 0.5.     (A4)[4 marks]

a.

\(-\frac{1}{3}(-1)^3 + \frac{5}{3}(-1)^2 – (-1) – 3 \)     (M1)

Note: Award (M1) for substitution of –1 into f (x)

= 0     (A1)(G2)[2 marks]

b.

(0, –3)     (A1)

OR

x = 0, y = –3     (A1)

Note: Award (A0) if brackets are omitted.[1 mark]

c.

\(f'(x) =  – {x^2} + \frac{{10}}{3}x – 1\)     (A1)(A1)(A1)

Note: Award (A1) for each correct term. Award (A1)(A1)(A0) at most if there are extra terms.[3 marks]

d.

\(f'( – 1) =  – {( – 1)^2} + \frac{{10}}{3}( – 1) – 1\)     (M1)

\(= -\frac{16}{3}\)     (AG)

Note: Award (M1) for substitution of x = –1 into correct derivative only. The final answer must be seen.[1 mark]

e.

f (–1) gives the gradient of the tangent to the curve at the point with x = –1.     (A1)(A1)

Note: Award (A1) for “gradient (of curve)”, (A1) for “at the point with x = –1”. Accept “the instantaneous rate of change of y” or “the (first) derivative”.[2 marks]

f.

\(y = – \frac{16}{3} x + c\)     (M1)

Note: Award (M1) for \(-\frac{16}{3}\) substituted in equation.

\(0 = – \frac{16}{3} \times (-1) + c \)

\(c = – \frac{16}{3}\)

\(y =  – \frac{{16}}{3}x – \frac{{16}}{3}\)     (A1)(G2)

Note: Accept y = –5.33x – 5.33.

OR

\((y – 0) = \frac{{-16}}{3}(x + 1)\)     (M1)(A1)(G2)

Note: Award (M1) for \( – \frac{{16}}{3}\) substituted in equation, (A1) for correct equation. Follow through from their answer to part (b). Accept y = –5.33 (x +1). Accept equivalent equations.[2 marks]

g.

(A1)(ft) for a tangent to their curve drawn.

(A1)(ft) for their tangent drawn at the point x = –1.     (A1)(ft)(A1)(ft)

Note: Follow through from their graph. The tangent must be a straight line otherwise award at most (A0)(A1).[2 marks]

h.

(i) \(a = \frac{1}{3}\)     (G1)

(ii) \(b = 3\)     (G1)

Note: If a and b are reversed award (A0)(A1).[2 marks]

i.

f (x) is increasing     (A1)[1 mark]

j.

Question

Consider the function \(g(x) = bx – 3 + \frac{1}{{{x^2}}},{\text{ }}x \ne 0\).

Write down the equation of the vertical asymptote of the graph of y = g(x) .[2]

a.

Write down g′(x) .[3]

b.

The line T is the tangent to the graph of y = g(x) at the point where x = 1. The gradient of T is 3.

Show that b = 5.[2]

c.

The line T is the tangent to the graph of y = g(x) at the point where x = 1. The gradient of T is 3.

Find the equation of T.[3]

d.

Using your graphic display calculator find the coordinates of the point where the graph of y = g(x) intersects the x-axis.[2]

e.

(i) Sketch the graph of y = g(x) for −2 ≤ x ≤ 5 and −15 ≤ y ≤ 25, indicating clearly your answer to part (e).

(ii) Draw the line T on your sketch.[6]

f.

Using your graphic display calculator find the coordinates of the local minimum point of y = g(x) .[2]

g.

Write down the interval for which g(x) is increasing in the domain 0 < x < 5 .[2]

h.
Answer/Explanation

Markscheme

x = 0     (A1)(A1)

Notes: Award (A1) for x=constant, (A1) for 0. Award (A0)(A0) if answer is not an equation.[2 marks]

a.

\(b – \frac{2}{{{x^3}}}\)     (A1)(A1)(A1)

Note: Award (A1) for b, (A1) for −2, (A1) for \(\frac{1}{{{x^3}}}\) (or x−3). Award at most (A1)(A1)(A0) if extra terms seen.[3 marks]

b.

\(3 = b – \frac{2}{{{{(1)}^3}}}\)     (M1)(M1)

Note: Award (M1) for substituting 1 into their gradient function, (M1) for equating their gradient function to 3.

b = 5     (AG)

Note: Award at most (M1)(A0) if final line is not seen or b does not equal 5.[2 marks]

c.

g(1) = 3 or (1, 3) (seen or implied from the line below)     (A1)

3 = 3 × 1 + c     (M1)

Note: Award (M1) for correct substitution of their point (1, 3) and gradient 3 into equation y = mx + c. Follow through from their point of tangency.

y = 3x     (A1)(ft)(G2)

OR

y − 3 = 3(x − 1)     (M1)(A1)(ft)(G2) 

Note: Award (M1) for substitution of gradient 3 and their point (1, 3) into y y1 = m(x x1), (A1)(ft) for correct substitutions. Follow through from their point of tangency. Award at most (A1)(M1)(A0)(ft) if further incorrect working seen.[3 marks]

d.

(−0.439, 0)   ((−0.438785…, 0))     (G1)(G1)

Notes: If no parentheses award at most (G1)(G0). Accept x = 0.439, y = 0.[2 marks]

e.

(i)

Award (A1) for labels and some indication of scale in the stated window.

Award (A1) for correct general shape (curve must be smooth and must not cross the y-axis)

Award (A1)(ft) for x-intercept consistent with their part (e).

Award (A1) for local minimum in the first quadrant.     (A1)(A1)(A1)(ft)(A1)

(ii) Tangent to curve drawn at approximately x = 1     (A1)(A1) 

Note: Award (A1) for a line tangent to curve approximately at x = 1. Must be a straight line for the mark to be awarded. Award (A1)(ft) for line passing through the origin. Follow through from their answer to part (d).[6 marks]

f.

(0.737, 2.53) ((0.736806…, 2.52604…))     (G1)(G1)

Notes: Do not penalize for lack of parentheses if already penalized in (e). Accept x = 0.737, y = 2.53.[2 marks]

g.

0.737 < x < 5  OR  (0.737;5)     (A1)(A1)(ft)

Notes: Award (A1) for correct strict or weak inequalities with x seen if the interval is given as inequalities, (A1)(ft) for 0.737 and 5 or their value from part (g).[2 marks]

h.
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