Question 5 (5 points)

Write an equation of the cosine function with amplitude 2 and period [tex]\(6 \pi\)[/tex].

A. [tex]\(y=-2 \cos \left(\frac{1}{6} x\right)\)[/tex]

B. [tex]\(y=-\frac{1}{2} \cos \left(\frac{1}{3} x\right)\)[/tex]

C. [tex]\(y=2 \cos \left(\frac{1}{3} x\right)\)[/tex]

D. [tex]\(y=\frac{1}{2} \cos \left(\frac{1}{6} x\right)\)[/tex]



Answer :

To write the equation of a cosine function with a given amplitude and period, it's important to recall the general form of the cosine function and how the amplitude and period are defined within it. The general form of a cosine function is:

[tex]\[ y = A \cos(Bx) \][/tex]

where
- [tex]\( A \)[/tex] represents the amplitude,
- [tex]\( B \)[/tex] affects the period of the function.

The amplitude [tex]\( A \)[/tex] is the maximum value of the function, which corresponds to the multiplier in front of the cosine function.

The period [tex]\( T \)[/tex] of the cosine function is given by the formula:

[tex]\[ T = \frac{2\pi}{B} \][/tex]

Given the requirements:
1. Amplitude = 2
2. Period = 6π

First, let's determine the amplitude.
- Here, the amplitude [tex]\( A \)[/tex] is directly given as 2. Therefore, we need the coefficient [tex]\( A \)[/tex] in front of the cosine function to be 2.

Next, let's determine the value of [tex]\( B \)[/tex] that will give us the desired period.
- The period [tex]\( T \)[/tex] is given by [tex]\( \frac{2\pi}{B} \)[/tex].
- We need this period to be [tex]\( 6\pi \)[/tex].

Setting up the equation for the period:
[tex]\[ 6\pi = \frac{2\pi}{B} \][/tex]

Solving for [tex]\( B \)[/tex]:
[tex]\[ B = \frac{2\pi}{6\pi} \][/tex]
[tex]\[ B = \frac{1}{3} \][/tex]

Thus, the cosine function that has an amplitude of 2 and a period of [tex]\( 6\pi \)[/tex] should have the form:

[tex]\[ y = 2 \cos\left(\frac{1}{3}x\right) \][/tex]

Reviewing the given options:

1. [tex]\( y = -2 \cos\left(\frac{1}{6} x\right) \)[/tex] - This has the correct amplitude but incorrect value of [tex]\( B \)[/tex].
2. [tex]\( y = -\frac{1}{2} \cos\left(\frac{1}{3} x\right) \)[/tex] - This has an incorrect amplitude, and while the period is correct, the overall sign is changed.
3. [tex]\( y = 2 \cos\left(\frac{1}{3} x\right) \)[/tex] - This has both the correct amplitude and period.
4. [tex]\( y = \frac{1}{2} \cos\left(\frac{1}{6} x\right) \)[/tex] - This has an incorrect amplitude and period.

Therefore, the correct equation is:

[tex]\[ y = 2 \cos\left(\frac{1}{3} x\right) \][/tex]

So, the correct choice is:
[tex]\[ y = 2 \cos\left(\frac{1}{3} x\right) \][/tex]

Thus, the answer is indeed:
[tex]\[ 3 \][/tex]