It is given that y1=sin⁡x−2y_1 = \sin x - 2 and y2=−3cos⁡2xy_2 = -3 \cos 2x.

  1. (i)

    State the amplitude and period, in degrees, of
    (a) y1y_1,
    (b) y2y_2.

  2. (ii)

    For the interval 0∘≤x≤360∘0^\circ \le x \le 360^\circ,
    solve the equation y1=y2y_1 = y_2,

  3. (iii)

    sketch, on the same diagram, the graphs of y1y_1 and y2y_2,

  4. (iv)

    find the set of values of xx for which y2−y1>0y_2 - y_1 > 0.

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Worked solution

8(i)

1. Determine the amplitude and period of $y_1 = \sin x - 2$

For y1=1sin⁡(1x)−2y_1 = 1\sin(1x) - 2:
Amplitude =∣1∣=1= |1| = 1
Period =360∘1=360∘= \frac{360^\circ}{1} = 360^\circ

2. Determine the amplitude and period of $y_2 = -3 \cos 2x$

For y2=−3cos⁡(2x)y_2 = -3\cos(2x):
Amplitude =∣−3∣=3= |-3| = 3
Period =360∘2=180∘= \frac{360^\circ}{2} = 180^\circ

Answer

(a) Amplitude = 1, Period = 360∘360^\circ
(b) Amplitude = 3, Period = 180∘180^\circ

8(ii)

1. Set up the equation and apply the double angle identity for cosine

sin⁡x−2=−3cos⁡2x\sin x - 2 = -3 \cos 2x
sin⁡x−2=−3(1−2sin⁡2x)\sin x - 2 = -3(1 - 2\sin^2 x)

2. Rearrange into a quadratic equation in terms of $\sin x$

sin⁡x−2=−3+6sin⁡2x\sin x - 2 = -3 + 6\sin^2 x
6sin⁡2x−sin⁡x−1=06\sin^2 x - \sin x - 1 = 0

3. Factorise and solve for $\sin x$

(3sin⁡x+1)(2sin⁡x−1)=0(3\sin x + 1)(2\sin x - 1) = 0
sin⁡x=−13 or sin⁡x=12\sin x = -\frac{1}{3} \text{ or } \sin x = \frac{1}{2}

4. Find the values of $x$ in the given interval

For sin⁡x=12\sin x = \frac{1}{2}:
Basic angle α=30∘\alpha = 30^\circ
x=30∘,180∘−30∘=150∘x = 30^\circ, 180^\circ - 30^\circ = 150^\circ

For sin⁡x=−13\sin x = -\frac{1}{3}:
Basic angle α=sin⁡−1(13)≈19.471∘\alpha = \sin^{-1}(\frac{1}{3}) \approx 19.471^\circ
x=180∘+19.471∘=199.471∘≈199.5∘x = 180^\circ + 19.471^\circ = 199.471^\circ \approx 199.5^\circ
x=360∘−19.471∘=340.529∘≈340.5∘x = 360^\circ - 19.471^\circ = 340.529^\circ \approx 340.5^\circ

Answer

x=30∘,150∘,199.5∘,340.5∘x = 30^\circ, 150^\circ, 199.5^\circ, 340.5^\circ

8(iii)

1. Identify key features of $y_1 = \sin x - 2$

Amplitude = 1, Period = 360∘360^\circ. The principal axis is y=−2y = -2. Maximum value = −1-1, minimum value = −3-3. Key points at x=0∘,90∘,180∘,270∘,360∘x = 0^\circ, 90^\circ, 180^\circ, 270^\circ, 360^\circ are (0,−2),(90,−1),(180,−2),(270,−3),(360,−2)(0, -2), (90, -1), (180, -2), (270, -3), (360, -2).

2. Identify key features of $y_2 = -3 \cos 2x$

Amplitude = 3, Period = 180∘180^\circ. The principal axis is y=0y = 0. Maximum value = 33, minimum value = −3-3. It is an inverted cosine graph. Key points for one cycle at x=0∘,45∘,90∘,135∘,180∘x = 0^\circ, 45^\circ, 90^\circ, 135^\circ, 180^\circ are (0,−3),(45,0),(90,3),(135,0),(180,−3)(0, -3), (45, 0), (90, 3), (135, 0), (180, -3). The second cycle repeats these values up to 360∘360^\circ.

3. Sketch both graphs on the same axes

Draw the axes for 0∘≤x≤360∘0^\circ \le x \le 360^\circ and −3≤y≤3-3 \le y \le 3. Plot the key points for y1y_1 and draw a smooth sine curve. Plot the key points for y2y_2 and draw two cycles of a smooth inverted cosine curve. Ensure the intersections align with the solutions from part (ii).

Answer

A sketch showing y1=sin⁡x−2y_1 = \sin x - 2 and y2=−3cos⁡2xy_2 = -3 \cos 2x on the same axes for 0∘≤x≤360∘0^\circ \le x \le 360^\circ.

8(iv)

1. Identify the intersection points from part (ii)

From part (ii), the solutions to y1=y2y_1 = y_2 are x=30∘,150∘,199.5∘,340.5∘x = 30^\circ, 150^\circ, 199.5^\circ, 340.5^\circ.

2. Use the graphs from part (iii) to determine where $y_2 > y_1$

The inequality y2−y1>0y_2 - y_1 > 0 is equivalent to y2>y1y_2 > y_1.
From the sketch in part (iii), the curve y2=−3cos⁡2xy_2 = -3 \cos 2x lies above the curve y1=sin⁡x−2y_1 = \sin x - 2 between the first and second intersection points, and between the third and fourth intersection points.
Therefore, the required set of values is 30∘<x<150∘30^\circ < x < 150^\circ or 199.5∘<x<340.5∘199.5^\circ < x < 340.5^\circ.

Answer

30∘<x<150∘30^\circ < x < 150^\circ or 199.5∘<x<340.5∘199.5^\circ < x < 340.5^\circ

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