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Date May Specimen paper Marks available 1 Reference code SPM.1.SL.TZ0.8
Level Standard Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Hence and Explain Question number 8 Adapted from N/A

Question

Let  f ( x ) = 1 3 x 3 + x 2 15 x + 17 .

The graph of f has horizontal tangents at the points where x = a and x = b , a < b .

Find f ( x ) .

[2]
a.

Find the value of a and the value of b .

[3]
b.

Sketch the graph of  y = f ( x ) .

[1]
c.i.

Hence explain why the graph of f has a local maximum point at x = a .

[1]
c.ii.

Find f ( b ) .

[3]
d.i.

Hence, use your answer to part (d)(i) to show that the graph of f has a local minimum point at x = b .

[1]
d.ii.

The normal to the graph of f at x = a and the tangent to the graph of f at x = b intersect at the point ( p , q ) .

 

Find the value of p and the value of q .

[5]
e.

Markscheme

f ( x ) = x 2 + 2 x 15      (M1)A1

 

[2 marks]

a.

correct reasoning that  f ( x ) = 0  (seen anywhere)    (M1)

x 2 + 2 x 15 = 0

valid approach to solve quadratic        M1

( x 3 ) ( x + 5 ) , quadratic formula

correct values for  x

3, −5

correct values for  a and b

a = −5 and  b = 3        A1

[3 marks]

b.

      A1

[1 mark]

c.i.

first derivative changes from positive to negative at  x = a       A1

so local maximum at  x = a      AG

[1 mark]

c.ii.

f ( x ) = 2 x + 2      A1

substituting their b into their second derivative     (M1)

f ( 3 ) = 2 × 3 + 2

f ( b ) = 8      (A1)

[3 marks]

d.i.

f ( b ) is positive so graph is concave up      R1

so local minimum at x = b       AG

[1 mark]

d.ii.

normal to f at  x = a is x = −5 (seen anywhere)          (A1)

attempt to find y -coordinate at their value of b           (M1)

f ( 3 ) = −10       (A1)

tangent at x = b has equation y = −10 (seen anywhere)         A1

intersection at (−5, −10)

p = −5 and q = −10        A1

[5 marks]

e.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.i.
[N/A]
c.ii.
[N/A]
d.i.
[N/A]
d.ii.
[N/A]
e.

Syllabus sections

Topic 5 —Calculus » SL 5.3—Differentiating polynomials, n E Z
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Topic 5 —Calculus » SL 5.4—Tangents and normal
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Topic 5 —Calculus

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