It is given that f(x)=2sin⁡x2f(x)=2\sin\frac{x}{2} and g(x)=3cos⁡x+1g(x)=3\cos x+1, where 0≤x≤2π0 \le x \le 2\pi.

  1. (a)

    State the period of f(x)f(x).

  2. (b)

    State the smallest value of f(x)f(x).

  3. (c)

    State the largest value of g(x)g(x).

  4. (d)

    Sketch on the same axes, the graphs of y=f(x)y=f(x) and y=g(x)y=g(x) for 0≤x≤2π0 \le x \le 2\pi. Label your graphs clearly.

  5. (e)

    The solutions to the equation f(x)=g(x)f(x)=g(x) for 0lexle2pi0 \\le x \\le 2\\pi are aa and bb, where a<ba<b. State, in terms of aa and bb, the range of values of xx for which f(x)>g(x)f(x)>g(x).

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Worked solution

10(a)

1. Identify the coefficient of $x$ and calculate the period

For f(x)=2sin⁡(12x)f(x) = 2\sin\left(\frac{1}{2}x\right), the coefficient of xx is 12\frac{1}{2}.
Period=2π1/2=4π\text{Period} = \frac{2\pi}{1/2} = 4\pi

Answer

4π4\pi

10(b)

1. Determine the smallest value of the function in the given domain

For 0≤x≤2π0 \le x \le 2\pi, the angle x2\frac{x}{2} ranges from 00 to π\pi.
In this interval, 0≤sin⁡x2≤10 \le \sin\frac{x}{2} \le 1.
Therefore, the smallest value of 2sin⁡x22\sin\frac{x}{2} is 2(0)=02(0) = 0.

Answer

00

10(c)

1. Determine the maximum value of the cosine function

The maximum value of cos⁡x\cos x is 11.

2. Calculate the largest value of $g(x)$

Largest value of g(x)=3(1)+1=4g(x) = 3(1) + 1 = 4

Answer

44

10(d)

1. Determine the key features of $y=f(x)=2\sin\frac{x}{2}$ for $0 \le x \le 2\pi$

Amplitude =2= 2. Period =2π1/2=4π= \frac{2\pi}{1/2} = 4\pi.
For 0≤x≤2π0 \le x \le 2\pi, the graph is half of a sine wave.
Key points: (0,0)(0,0), maximum at (π,2)(\pi, 2), and (2π,0)(2\pi, 0).

2. Determine the key features of $y=g(x)=3\cos x+1$ for $0 \le x \le 2\pi$

Amplitude =3= 3. Period =2π= 2\pi. Vertical shift =+1= +1.
Maximum value =3+1=4= 3+1=4, minimum value =−3+1=−2= -3+1=-2.
Key points: (0,4)(0,4), (π2,1)(\frac{\pi}{2}, 1), (π,−2)(\pi, -2), (3π2,1)(\frac{3\pi}{2}, 1), (2π,4)(2\pi, 4).

3. Sketch both graphs on the same axes and label them

Draw the axes. Plot the key points for f(x)f(x) and draw a smooth curve. Plot the key points for g(x)g(x) and draw a smooth curve. Label the curves y=f(x)y=f(x) and y=g(x)y=g(x).

Answer

A sketch showing y=2sin⁡x2y=2\sin\frac{x}{2} as a half-sine wave from (0,0)(0,0) to (2π,0)(2\pi,0) with a peak at (π,2)(\pi,2), and y=3cos⁡x+1y=3\cos x+1 as a full cosine wave shifted up by 1, starting at (0,4)(0,4), passing through (π2,1)(\frac{\pi}{2},1), reaching a minimum at (π,−2)(\pi,-2), passing through (3π2,1)(\frac{3\pi}{2},1), and ending at (2π,4)(2\pi,4). Both curves are labelled.

10(e)

1. Interpret the inequality $f(x) > g(x)$ graphically

The inequality f(x)>g(x)f(x) > g(x) corresponds to the region where the graph of y=f(x)y = f(x) lies above the graph of y=g(x)y = g(x).

2. Determine the range of $x$ using the intersection points

From the sketch in part (d), the curve y=2sin⁡x2y = 2\sin\frac{x}{2} starts at (0,0)(0,0), reaches a maximum of 22 at x=πx=\pi, and ends at (2π,0)(2\pi, 0). The curve y=3cos⁡x+1y = 3\cos x + 1 starts at (0,4)(0,4), goes down to a minimum of −2-2 at x=πx=\pi, and goes back up to (2π,4)(2\pi, 4).
The graphs intersect at x=ax=a and x=bx=b (where a<ba < b). Between these two points, the sine curve f(x)f(x) is above the cosine curve g(x)g(x).
Therefore, f(x)>g(x)f(x) > g(x) for a<x<ba < x < b.

Answer

a<x<ba < x < b

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