Hong Kong
Stage 4 - Stage 5

# Key features of sine and cosine curves (radians)

Lesson

When looking at the unit circle, that is a circle with the origin at the centre and unit radius, the coordinates of any point on that circle can be described using trigonometry. Specifically a point on the circle at an angle of $\theta$θ anticlockwise from the $x$x-axis has coordinates $\left(\cos\theta,\sin\theta\right)$(cosθ,sinθ).

 The unit circle

As we move through different values of $\theta$θ the value of $\cos\theta$cosθ and $\sin\theta$sinθ move accordingly between $-1$1 and $1$1. If we plot the values of $\cos\theta$cosθ and $\sin\theta$sinθ according to different values of $\theta$θ on the unit circle, we get the following graphs:

 $y=\cos\theta$y=cosθ

 $y=\sin\theta$y=sinθ

Consequently, the graphs of $y=\cos\theta$y=cosθ and $y=\sin\theta$y=sinθ have many properties. Each graph demonstrates repetition. We call the graphs of $y=\cos\theta$y=cosθ and $y=\sin\theta$y=sinθ cyclical and define a cycle as any section of the graph that can be translated to complete the rest of the graph. We also define the period as the length of one cycle. For both graphs, the period is $2\pi$2π.

 An example of a cycle

Because of the oscillating behaviour, both graphs have regions where the curve is increasing and decreasing. Remember that we say the graph of a particular curve is increasing if the $y$y-values increase as the $x$x-values increase. Similarly, we say the graph is decreasing if the $y$y-values decrease as the $x$x-values increase.

 An example of where $y=\sin x$y=sinx is decreasing

In addition, the height of each graph stays between $y=-1$y=1 and $y=1$y=1 for all values of $\theta$θ, since each coordinate of a point on the unit circle can be at most $1$1 unit from the origin.

#### Practice questions

##### QUESTION 1

Consider the curve $y=\sin x$y=sinx drawn below and answer the following questions.

1. What is the $y$y-intercept? Give your answer as coordinates in the form $\left(a,b\right)$(a,b).

2. What is the maximum $y$y-value?

3. What is the minimum $y$y-value?

##### question 2

Consider the curve $y=\cos x$y=cosx drawn below and determine whether the following statements are true or false.

1. The graph of $y=\cos x$y=cosx is cyclic.

True

A

False

B
2. As $x$x approaches infinity, the height of the graph for $y=\cos x$y=cosx approaches infinity.

True

A

False

B
3. The graph of $y=\cos x$y=cosx is increasing between $x=-\frac{\pi}{2}$x=π2 and $x=0$x=0.

False

A

True

B

##### question 3

Consider the curve $y=\sin x$y=sinx drawn below and answer the following questions.

1. If one cycle of the graph of $y=\sin x$y=sinx starts at $x=0$x=0, when does the next cycle start?

2. In which of the following regions is the graph of $y=\sin x$y=sinx decreasing? Select all that apply.

$-\frac{\pi}{2}π2<x<π2 A$\frac{\pi}{2}π2<x<3π2

B

$-\frac{5\pi}{2}5π2<x<3π2 C$-\frac{3\pi}{2}3π2<x<π2

D
3. What is the $x$x-value of the $x$x-intercept in the region \$00<x<2π?