Graph of Sine, Cosine and Tangent Functions

6.3   Graph of Sine, Cosine and Tangent Functions
 
  • The graphs of \(y = \text{sin } x\) and \(y = \text{cos } x\) are sinusoidal and have the following properties:

     
   
(a)

The maximum value is \(1\) while the minimum value is \(-1\),

so the amplitude of the graph is \(1\) unit.

   
(b)

The graph repeats itself every \(360^{\circ}\text{ or }2\pi \text{ rad}\),

so \(360^{\circ}\text{ or }2\pi \text{ rad}\) is the period for both graphs.

   
     
 
  • The graph \(y = \text{tan } x\) is not sinusoidal and the properties are as follows:

     
   
(a)

This graph has no maximum or minimum value.

   
(b)

The graph repeats itself every \(180^{\circ}\text{ or }\pi \text{ rad}\) interval,

so the period of a tangent graph is \(180^{\circ}\text{ or }\pi \text{ rad}\).

   
(c)

The function \(y = \text{tan } x\) is not defined at \(x = 90^{\circ} \text{ and } x = 270^{\circ}\).

The curve approaches the line but does nottouch the line.

This line is called an asymptote.

   
     
 
  • The graphs for these three functions are as follows:
 
     
   Graph \(y = \text{cos } x\) for \(-2\pi \leqslant x \leqslant 2\pi\)   
 
(a)

Amplitude \(=1\)

  • The maximum value of \(y=1\)
  • The minimum value of  \(y=-1\)
(b) Period \(=360^{\circ}\text{ or }2\pi \) 
(c)

\(x\)-intercepts:

\(-\dfrac{3}{2}\pi, \ -\dfrac{1}{2}\pi, \ \dfrac{1}{2}\pi, \ \dfrac{3}{2}\pi\)

(d) \(y\)-intercepts: \(1\) 
   
 
     
  

 Graph \(y = \text{sin } x\) for \(-2\pi \leqslant x \leqslant 2\pi\)

  
 
(a)

 Amplitude \(=1\)

  • The maximum value of \(y=1\)
  • The minimum value of \(y=-1\) 
(b) Period \(=360^{\circ}\text{ atau }2\pi \) 
(c)

\(x\)-intercepts: 

\(-2\pi, \ -\pi, \ 0, \ \pi, \ 2\pi \)

(d)  \(y\)-intercepts: \(0\)
   
 
     
   Graph \(y = \text{tan } x\) for \(-2\pi \leqslant x \leqslant 2\pi\)   
 
(a)

No amplitude

  • There are no maximum and minimum values of \(y\) 
(b) Period \(=180^{\circ}\text{ or }\pi \) 
(c)

\(x\)-asymptotes:

\(-\dfrac{3}{2}\pi, \ -\dfrac{1}{2}\pi, \ \dfrac{1}{2}\pi, \ \dfrac{3}{2}\pi\)

(d)

\(x\)-intercepts:

\(-2\pi, \ -\pi, \ 0, \ \pi, \ 2\pi \)

(e)  \(y\)-intercepts: \(0\)
   
 
 
 
  • The values of \(a\), \(b\) and \(c\) in the function \(y = a \text{ sin } bx + c\) affect the amplitude, the period and the position of the graph

  • The effects of changing the values of \(a\)\(b\) and \(c\) on the graph can be summarised as follows:

 
Change in Effects
     
  \(a\)  
     

 

     
  

The maximum and minimum values of the graphs (except for the graph of \(y = \text{tan } x\) where there is no maximum or minimum value)

  
     
     
  \(b\)  
     

 

     
  

Number of cycles in the range \(0^{\circ} \leqslant x \leqslant 360^{\circ} \text{ or }0^{\circ} \leqslant x \leqslant 2\pi\):

  • Graphs \(y = \text{sin } x\) and \(y = \text{cos } x\) \(\begin{pmatrix} \text{period } = \dfrac{360^{\circ}}{b} \text{ or } \dfrac{2}{b}\pi\end{pmatrix}\)
  • Graph \(y = \text{tan } x \ \begin{pmatrix} \text{period } = \dfrac{180^{\circ}}{b} \text{ or } \dfrac{1}{b}\pi\end{pmatrix}\)  
  
     
     
  \(c\)  
     
     
   The position of the graph with reference to the \(x\)-axis as compared to the position of the basic graph   
     
 

Example:

 
Example
     
  

State the cosine function represented by the graph above.

  
     
  Solution:  
     
 

Note that the amplitude is \(4\).

So, \(a=4.\)

Two cycles in the range of \(0^{\circ} \leqslant x \leqslant 2\pi\).

The period is \(\pi\), that is, \(\dfrac{2\pi}{b} = \pi, \text{ so }b=2.\)

Hence, the graph represents \(y= 4\text{ cos }2x\).