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Date May Specimen paper Marks available 3 Reference code SPM.1.SL.TZ0.8
Level Standard Level Paper Paper 1 (without calculator) Time zone Time zone 0
Command term Find Question number 8 Adapted from N/A

Question

Let f(x)=13x3+x215x+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)=x2+2x15     (M1)A1

 

[2 marks]

a.

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

x2+2x15=0

valid approach to solve quadratic        M1

(x3)(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)=2x+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
Topic 5 —Calculus » SL 5.8—Testing for max and min, optimisation. Points of inflexion
Topic 5 —Calculus

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