Find an expression for r in terms of θ.

Find the possible values of r.

Show that the sum of the infinite sequence is $\frac{{54}}{{2 + {\text{cos}}\,\left( {2\theta } \right)}}$.

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

valid approach     (M1)

eg   $\frac{{{u_2}}}{{{u_1}}},\,\,\frac{{{u_1}}}{{{u_2}}}$

$r = \frac{{12\,{{\sin }^2}\,\theta }}{{18}}\left( { = \frac{{2\,{{\sin }^2}\,\theta }}{3}} \right)$      A1 N2

[2 marks]

recognizing that sinθ is bounded      (M1)

eg    0 ≤ sin² θ ≤ 1, −1 ≤ sinθ ≤ 1, −1 < sinθ < 1

0 < r ≤ $\frac{2}{3}$      A2 N3

Note: If working shown, award M1A1 for correct values with incorrect inequality sign(s).
If no working shown, award N1 for correct values with incorrect inequality sign(s).

[3 marks]

correct substitution into formula for infinite sum       A1

eg  $\frac{{18}}{{1 - \frac{{2\,{\text{si}}{{\text{n}}^2}\,\theta }}{3}}}$

evidence of choosing an appropriate rule for cos 2θ (seen anywhere)         (M1)

eg   cos 2θ = 1 − 2 sin² θ

correct substitution of identity/working (seen anywhere)      (A1)

eg   $\frac{{18}}{{1 - \frac{2}{3}\left( {\frac{{1 - {\text{cos}}\,2\theta }}{2}} \right)}},\,\,\frac{{54}}{{3 - 2\left( {\frac{{1 - {\text{cos}}\,2\theta }}{2}} \right)}},\,\,\frac{{18}}{{\frac{{3 - 2\,{\text{si}}{{\text{n}}^2}\,\theta }}{3}}}$

correct working that clearly leads to the given answer       A1

eg  $\frac{{18 \times 3}}{{2 + \left( {1 - 2\,{\text{si}}{{\text{n}}^2}\,\theta } \right)}},\,\,\frac{{54}}{{3 - \left( {1 - {\text{cos}}\,2\theta } \right)}}$

$\frac{{54}}{{2 + {\text{cos}}\left( {2\theta } \right)}}$    AG N0

[4 marks]

 

METHOD 1 (using differentiation)

recognizing $\frac{{{\text{d}}{S_\infty }}}{{{\text{d}}\theta }} = 0$ (seen anywhere)       (M1)

finding any correct expression for $\frac{{{\text{d}}{S_\infty }}}{{{\text{d}}\theta }}$       (A1)

eg  $\frac{{0 - 54 \times \left( { - 2\,{\text{sin}}\,2\,\theta } \right)}}{{{{\left( {2 + {\text{cos}}\,2\,\theta } \right)}^2}}},\,\, - 54{\left( {2 + {\text{cos}}\,2\,\theta } \right)^{ - 2}}\,\left( { - 2\,{\text{sin}}\,2\,\theta } \right)$

correct working       (A1)

eg  sin 2θ = 0

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

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

2θ = $\pi$, 3$\pi$ (accept values in degrees)

both correct answers $\theta  = \frac{\pi }{2},\,\frac{{3\pi }}{2}$      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 r = $\frac{2}{3}$

correct working      (A1)

eg  ${\text{cos}}\,2\,\theta  =  - 1,\,\,\frac{2}{3}\,{\text{si}}{{\text{n}}^2}\,\theta  = \frac{2}{3},\,\,{\text{si}}{{\text{n}}^2}\theta  = 1$

EITHER (using cos 2θ)

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

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

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

2θ = $\pi$, 3$\pi$ (accept values in degrees)

OR (using sinθ)

sinθ = ±1     (A1)

sin⁻¹(1) = $\frac{\pi }{2}$ (accept values in degrees) (seen anywhere)      A1

THEN

both correct answers $\theta  = \frac{\pi }{2},\,\frac{{3\pi }}{2}$       A1 N4

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

[6 marks]

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