Question

A cosine function is a reflection of its parent function over the x-axis. The amplitude of the function is 11, the vertical shift is 9 units down, and the period of the function is 7pi/12. The graph of the function does not show a phase shift. What is the equation of the cosine function described?
Fill in the blanks using values from this list: -20, -11, -9, -2, 2, 9, 11, 20, 7pi/12,12pi/7,7/24,24/7

f(x)=___cos(_____x)______

Answer 1

We have been given that a cosine function is a reflection of its parent function over the x-axis. The amplitude of the function is 11, the vertical shift is 9 units down, and the period of the function is $\frac{7\pi}{12}$. The graph of the function does not show a phase shift. We are asked to write the equation of our function.

We know that general form a cosine function is $y=A\cos(b(x-c))-d$, where,

A = Amplitude,

$\frac{2\pi}{b}$ = Period,

c = Horizontal shift,

d = Vertical shift.    

The equation of parent cosine function is $y=\cos(x)$. Since function is reflected about x-axis, so our function will be $y=-\cos(x)$.

Let us find the value of b.

$\frac{2\pi}{b}=\frac{7\pi}{12}$

$7\pi\cdot b=24\pi$

$\frac{7\pi\cdot b}{7\pi}=\frac{24\pi}{7\pi}$

$b=\frac{24}{7}$

Upon substituting our given values in general cosine function, we will get:

$f(x)=-11\cos(\frac{24}{7}x)-9$

Therefore, our required function would be $f(x)=-11\cos(\frac{24}{7}x)-9$.