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Identifying the variations of a sinusoidal function from an equation

Determine the midline, amplitude, period, and phase shift of the function y = 3 sin ( 2 x ) + 1.

Let’s begin by comparing the equation to the general form y = A sin ( B x C ) + D .

A = 3 , so the amplitude is | A | = 3.

Next, B = 2 , so the period is P = 2 π | B | = 2 π 2 = π .

There is no added constant inside the parentheses, so C = 0 and the phase shift is C B = 0 2 = 0.

Finally, D = 1 , so the midline is y = 1.

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Determine the midline, amplitude, period, and phase shift of the function y = 1 2 cos ( x 3 π 3 ) .

midline: y = 0 ; amplitude: | A | = 1 2 ; period: P = 2 π | B | = 6 π ; phase shift: C B = π

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Identifying the equation for a sinusoidal function from a graph

Determine the formula for the cosine function in [link] .

A graph of -0.5cos(x)+0.5. The graph has an amplitude of 0.5. The graph has a period of 2pi. The graph has a range of [0, 1]. The graph is also reflected about the x-axis from the parent function cos(x).

To determine the equation, we need to identify each value in the general form of a sinusoidal function.

y = A sin ( B x C ) + D y = A cos ( B x C ) + D

The graph could represent either a sine or a cosine function    that is shifted and/or reflected. When x = 0 , the graph has an extreme point, ( 0 , 0 ) . Since the cosine function has an extreme point for x = 0 , let us write our equation in terms of a cosine function.

Let’s start with the midline. We can see that the graph rises and falls an equal distance above and below y = 0.5. This value, which is the midline, is D in the equation, so D = 0.5.

The greatest distance above and below the midline is the amplitude. The maxima are 0.5 units above the midline and the minima are 0.5 units below the midline. So | A | = 0.5. Another way we could have determined the amplitude is by recognizing that the difference between the height of local maxima and minima is 1, so | A | = 1 2 = 0.5. Also, the graph is reflected about the x -axis so that A = 0.5.

The graph is not horizontally stretched or compressed, so B = 1; and the graph is not shifted horizontally, so C = 0.

Putting this all together,

g ( x ) = 0.5 cos ( x ) + 0.5
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Determine the formula for the sine function in [link] .

A graph of sin(x)+2. Period of 2pi, amplitude of 1, and range of [1, 3].

f ( x ) = sin ( x ) + 2

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Identifying the equation for a sinusoidal function from a graph

Determine the equation for the sinusoidal function in [link] .

A graph of 3cos(pi/3x-pi/3)-2. Graph has amplitude of 3, period of 6, range of [-5,1].

With the highest value at 1 and the lowest value at −5 , the midline will be halfway between at −2. So D = −2.

The distance from the midline to the highest or lowest value gives an amplitude of | A | = 3.

The period of the graph is 6, which can be measured from the peak at x = 1 to the next peak at x = 7 , or from the distance between the lowest points. Therefore, P = 2 π | B | = 6. Using the positive value for B , we find that

B = 2 π P = 2 π 6 = π 3

So far, our equation is either y = 3 sin ( π 3 x C ) 2 or y = 3 cos ( π 3 x C ) 2. For the shape and shift, we have more than one option. We could write this as any one of the following:

  • a cosine shifted to the right
  • a negative cosine shifted to the left
  • a sine shifted to the left
  • a negative sine shifted to the right

While any of these would be correct, the cosine shifts are easier to work with than the sine shifts in this case because they involve integer values. So our function becomes

y = 3 cos ( π 3 x π 3 ) 2  or  y = 3 cos ( π 3 x + 2 π 3 ) 2

Again, these functions are equivalent, so both yield the same graph.

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Questions & Answers

what is a function?
CJ Reply
I want to learn about the law of exponent
Quera Reply
explain this
Hinderson Reply
what is functions?
Angel Reply
A mathematical relation such that every input has only one out.
yes..it is a relationo of orders pairs of sets one or more input that leads to a exactly one output.
Is a rule that assigns to each element X in a set A exactly one element, called F(x), in a set B.
If the plane intersects the cone (either above or below) horizontally, what figure will be created?
Feemark Reply
can you not take the square root of a negative number
Sharon Reply
No because a negative times a negative is a positive. No matter what you do you can never multiply the same number by itself and end with a negative
Actually you can. you get what's called an Imaginary number denoted by i which is represented on the complex plane. The reply above would be correct if we were still confined to the "real" number line.
Suppose P= {-3,1,3} Q={-3,-2-1} and R= {-2,2,3}.what is the intersection
Elaine Reply
can I get some pretty basic questions
Ama Reply
In what way does set notation relate to function notation
is precalculus needed to take caculus
Amara Reply
It depends on what you already know. Just test yourself with some precalculus questions. If you find them easy, you're good to go.
the solution doesn't seem right for this problem
Mars Reply
what is the domain of f(x)=x-4/x^2-2x-15 then
Conney Reply
x is different from -5&3
All real x except 5 and - 3
how to prroved cos⁴x-sin⁴x= cos²x-sin²x are equal
jeric Reply
Don't think that you can.
By using some imaginary no.
how do you provided cos⁴x-sin⁴x = cos²x-sin²x are equal
jeric Reply
What are the question marks for?
Someone should please solve it for me Add 2over ×+3 +y-4 over 5 simplify (×+a)with square root of two -×root 2 all over a multiply 1over ×-y{(×-y)(×+y)} over ×y
Abena Reply
For the first question, I got (3y-2)/15 Second one, I got Root 2 Third one, I got 1/(y to the fourth power) I dont if it's right cause I can barely understand the question.
Is under distribute property, inverse function, algebra and addition and multiplication function; so is a combined question
find the equation of the line if m=3, and b=-2
Ashley Reply
graph the following linear equation using intercepts method. 2x+y=4
ok, one moment
how do I post your graph for you?
it won't let me send an image?
also for the first one... y=mx+b so.... y=3x-2
y=mx+b you were already given the 'm' and 'b'. so.. y=3x-2
Please were did you get y=mx+b from
y=mx+b is the formula of a straight line. where m = the slope & b = where the line crosses the y-axis. In this case, being that the "m" and "b", are given, all you have to do is plug them into the formula to complete the equation.
thanks Tommy
0=3x-2 2=3x x=3/2 then . y=3/2X-2 I think
co ordinates for x x=0,(-2,0) x=1,(1,1) x=2,(2,4)
Practice Key Terms 5

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Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
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