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Date May 2018 Marks available 2 Reference code 18M.1.SL.TZ1.S_10
Level Standard Level Paper Paper 1 (without calculator) Time zone Time zone 1
Command term Find Question number S_10 Adapted from N/A

Question

The first two terms of an infinite geometric sequence are u1 = 18 and u2 = 12sin2 θ , where 0 < θ < 2ππ , and θππ.

Find an expression for r in terms of θ.

[2]
a.i.

Find the values of θ which give the greatest value of the sum.

[6]
c.

Markscheme

valid approach     (M1)

eg   u2u1,u1u2u2u1,u1u2

r=12sin2θ18(=2sin2θ3)r=12sin2θ18(=2sin2θ3)      A1 N2

[2 marks]

a.i.

 

METHOD 1 (using differentiation)

recognizing dSdθ=0dSdθ=0 (seen anywhere)       (M1)

finding any correct expression for dSdθdSdθ       (A1)

eg  054×(2sin2θ)(2+cos2θ)2,54(2+cos2θ)2(2sin2θ)054×(2sin2θ)(2+cos2θ)2,54(2+cos2θ)2(2sin2θ)

correct working       (A1)

eg  sin 2θ = 0

any correct value for sin−1(0) (seen anywhere)       (A1)

eg  0, ππ, … , sketch of sine curve with x-intercept(s) marked both correct values for 2θ (ignore additional values)      (A1)

2θ ππ, 3ππ (accept values in degrees)

both correct answers θ=π2,3π2θ=π2,3π2      A1 N4

Note: Award A0 if either or both correct answers are given in degrees.
Award A0 if additional values are given.

 

METHOD 2 (using denominator)

recognizing when S is greatest      (M1)

eg 2 + cos 2θ is a minimum, 1−r is smallest
correct working      (A1)

eg  minimum value of 2 + cos 2θ is 1, minimum r2323

correct working      (A1)

eg  cos2θ=1,23sin2θ=23,sin2θ=1cos2θ=1,23sin2θ=23,sin2θ=1

EITHER (using cos 2θ)

any correct value for cos−1(−1) (seen anywhere)      (A1)

eg  ππ, 3ππ, … (accept values in degrees), sketch of cosine curve with x-intercept(s) marked

both correct values for 2θ  (ignore additional values)      (A1)

2θ ππ, 3ππ (accept values in degrees)

OR (using sinθ)

sinθ = ±1     (A1)

sin−1(1) = π2π2 (accept values in degrees) (seen anywhere)      A1

THEN

both correct answers θ=π2,3π2θ=π2,3π2       A1 N4

Note: Award A0 if either or both correct answers are given in degrees.
Award A0 if additional values are given.

[6 marks]

c.

Examiners report

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

Syllabus sections

Topic 1—Number and algebra » SL 1.3—Geometric sequences and series
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Topic 1—Number and algebra » SL 1.8—Sum of infinite geo sequence
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