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Date May 2019 Marks available 2 Reference code 19M.2.SL.TZ2.S_4
Level Standard Level Paper Paper 2 Time zone Time zone 2
Command term Find Question number S_4 Adapted from N/A

Question

OAB is a sector of the circle with centre O and radius r, as shown in the following diagram.

The angle AOB is θ radians, where 0<θ<π2.

The point C lies on OA and OA is perpendicular to BC.

Show that OC=rcosθ.

[1]
a.

Find the area of triangle OBC in terms of r and θ.

[2]
b.

Given that the area of triangle OBC is 35 of the area of sector OAB, find θ.

[4]
c.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

cosθ=OCr     A1

OC=rcosθ   AG N0

[1 mark]

a.

valid approach    (M1)

eg   12OC×OBsinθ ,  BC=rsinθ12rcosθ×BC ,  12rsinθ×OC

area =12r2sinθcosθ  (=14r2sin(2θ))  (must be in terms of r and θ)      A1 N2

[2 marks]

b.

valid attempt to express the relationship between the areas (seen anywhere)        (M1)

eg   OCB = 35OBA ,  12r2sinθcosθ=35×12r2θ ,  14r2sin2θ=310r2θ

correct equation in terms of θ only      A1

eg   sinθcosθ=35θ ,  14sin2θ=310θ

valid attempt to solve their equation        (M1)

eg    sketch,  −0.830017,  0

0.830017

θ = 0.830      A1 N2

Note: Do not award final A1 if additional answers given.

[4 marks]

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Topic 3— Geometry and trigonometry » SL 3.4—Circle: radians, arcs, sectors
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Topic 2—Functions » SL 2.10—Solving equations graphically and analytically
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Topic 3— Geometry and trigonometry

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