The function ff is defined, for 0≤x≤2π0 \le x \le 2\pi, by f(x)=1−2cos⁡2xf(x) = 1 - 2 \cos 2x.

  1. (a)

    State the period and amplitude of ff.

  2. (b)

    Sketch the graph of y=f(x)y = f(x) for 0≤x≤2π0 \le x \le 2\pi.

  3. (c)

    On the same diagram in part (b), sketch the graph of y=4xπ−2y = \frac{4x}{\pi} - 2.

  4. (d)

    Hence, state the number of solutions, 0≤x≤2π0 \le x \le 2\pi, to the equation
    2cos⁡2x−3+4xπ=0.2 \cos 2x - 3 + \frac{4x}{\pi} = 0.

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Worked solution

11(a)

1. Determine the period of the function

The function is f(x)=1−2cos⁡2xf(x) = 1 - 2 \cos 2x. The coefficient of xx is 22.
Period=2π2=π\text{Period} = \frac{2\pi}{2} = \pi

2. Determine the amplitude of the function

The coefficient of the cosine term is −2-2.
Amplitude=∣−2∣=2\text{Amplitude} = |-2| = 2

Answer

Period = π\pi, Amplitude = 22

11(b)

1. Determine the maximum and minimum values of the graph

The maximum value of −2cos⁡2x-2\cos 2x is 22, so the maximum value of f(x)f(x) is 1+2=31 + 2 = 3.
The minimum value of −2cos⁡2x-2\cos 2x is −2-2, so the minimum value of f(x)f(x) is 1−2=−11 - 2 = -1.

2. Identify the number of cycles and key $x$-coordinates

The period is π\pi, so there are two full cycles in the interval 0≤x≤2π0 \le x \le 2\pi.
The key points for the first cycle occur at intervals of π4\frac{\pi}{4}: 0,π4,π2,3π4,π0, \frac{\pi}{4}, \frac{\pi}{2}, \frac{3\pi}{4}, \pi.

3. Sketch the graph

Plot the key points:
(0,−1),(π4,1),(π2,3),(3π4,1),(π,−1)(0, -1), (\frac{\pi}{4}, 1), (\frac{\pi}{2}, 3), (\frac{3\pi}{4}, 1), (\pi, -1)
(5π4,1),(3π2,3),(7π4,1),(2π,−1)(\frac{5\pi}{4}, 1), (\frac{3\pi}{2}, 3), (\frac{7\pi}{4}, 1), (2\pi, -1)
Draw a smooth curve through these points.

Answer

A sketch of y=1−2cos⁡2xy = 1 - 2\cos 2x showing two full cycles of an inverted cosine wave starting at (0,−1)(0, -1), reaching a maximum of 33 at x=π2x = \frac{\pi}{2} and 3π2\frac{3\pi}{2}, a minimum of −1-1 at x=πx = \pi and 2π2\pi, and crossing the principal axis y=1y = 1 at x=π4,3π4,5π4,7π4x = \frac{\pi}{4}, \frac{3\pi}{4}, \frac{5\pi}{4}, \frac{7\pi}{4}.

11(c)

1. Identify the type of graph and find key points to plot

The equation y=4xπ−2y = \frac{4x}{\pi} - 2 represents a straight line.
When x=0x = 0, y=−2y = -2.
When x=π2x = \frac{\pi}{2}, y=4(π/2)π−2=0y = \frac{4(\pi/2)}{\pi} - 2 = 0.
When x=πx = \pi, y=4ππ−2=2y = \frac{4\pi}{\pi} - 2 = 2.
When x=2πx = 2\pi, y=4(2π)π−2=6y = \frac{4(2\pi)}{\pi} - 2 = 6.

2. Sketch the line on the same diagram

Draw a straight line passing through the points (0,−2)(0, -2), (π2,0)(\frac{\pi}{2}, 0), (π,2)(\pi, 2), and (2π,6)(2\pi, 6) on the axes from part (b).

Answer

A straight line passing through (0,−2)(0, -2), (π2,0)(\frac{\pi}{2}, 0), (π,2)(\pi, 2), and (2π,6)(2\pi, 6).

11(d)

1. Rearrange the given equation to match the functions sketched in parts (b) and (c).

2cos⁡2x−3+4xπ=02 \cos 2x - 3 + \frac{4x}{\pi} = 0
−2cos⁡2x=4xπ−3-2 \cos 2x = \frac{4x}{\pi} - 3
1−2cos⁡2x=4xπ−21 - 2 \cos 2x = \frac{4x}{\pi} - 2

2. Relate the rearranged equation to the number of intersections of the graphs.

The equation is now in the form f(x)=4xπ−2f(x) = \frac{4x}{\pi} - 2. The number of solutions is the number of points of intersection between the curve y=1−2cos⁡2xy = 1 - 2 \cos 2x and the line y=4xπ−2y = \frac{4x}{\pi} - 2 for 0≤x≤2π0 \le x \le 2\pi.

3. Determine the number of intersections from the properties of the graphs.

Checking key points:
At x=0x = 0, curve y=−1y = -1, line y=−2y = -2 (curve is above line).
At x=π2x = \frac{\pi}{2}, curve y=3y = 3, line y=0y = 0 (curve is above line).
At x=3π4x = \frac{3\pi}{4}, curve y=1−2cos⁡(3π2)=1y = 1 - 2\cos\left(\frac{3\pi}{2}\right) = 1, line y=4π(3π4)−2=1y = \frac{4}{\pi}\left(\frac{3\pi}{4}\right) - 2 = 1. They intersect exactly here.
At x=πx = \pi, curve y=−1y = -1, line y=2y = 2 (line is above curve).
For x>3π4x > \frac{3\pi}{4}, the line continues to rise (up to y=6y=6 at x=2πx=2\pi) while the curve's maximum is 33, so they do not intersect again.
Thus, there is exactly 11 point of intersection.

Answer

11

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