Sketch the Graph of ψ(x) = cos 3x
Question:
Sketch the graph of the following trigonometric function :
\[ \psi(x)=\cos3x \]
Solution:
We know that
\[ y=\cos x \]
is the standard cosine curve.
In the function
\[ y=\cos3x \]
the angle is multiplied by \(3\). Therefore the graph oscillates faster.
Important properties:
- Amplitude \(=1\)
- Period \(=\dfrac{2\pi}{3}\)
- Range \(-1 \le y \le 1\)
Thus one complete cosine wave is obtained in the interval
\[ 0 \le x \le \frac{2\pi}{3} \]
Now calculate some important points:
\[ \begin{aligned} x=0 &\Rightarrow y=\cos0=1\\[8pt] x=\frac{\pi}{6} &\Rightarrow y=\cos\frac{\pi}{2}=0\\[8pt] x=\frac{\pi}{3} &\Rightarrow y=\cos\pi=-1\\[8pt] x=\frac{\pi}{2} &\Rightarrow y=\cos\frac{3\pi}{2}=0\\[8pt] x=\frac{2\pi}{3} &\Rightarrow y=\cos2\pi=1 \end{aligned} \]
Thus the curve passes through the points
\[ (0,1),\quad \left(\frac{\pi}{6},0\right),\quad \left(\frac{\pi}{3},-1\right),\quad \left(\frac{\pi}{2},0\right),\quad \left(\frac{2\pi}{3},1\right) \]
Plot these points and draw a smooth cosine curve through them.
Hence, the required graph is shown above.