If you're seeing this message, it means we're having trouble loading external resources for Khan Academy.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Graphs of trig functions

## Example: Graph of cosine

Basic interpretation of the graph of the cosine function
Back

## Example: Graph of cosine

Discussion and questions for this video
What is exactly happening at1:20 and why is 'y' taken as equal to cosQ?
Comment
im learning this myself and this is what i have so far.

"y" is taken as the equal to cos(theta) because that is what you are measuring. remember the "x" axis is actually the "theta" axis. you you are graphing where cos(theta) is when theta is at a specified value. so x-axis=theta and y-axis=cos(theta). if you were trying to find the sin(theta), this would be a different graph, then that would make y-axis=sin(theta).

i was at first confused from the fact that the coordinates were supposed to be (cos, sin), but then realized that this only applied to the unit circle.

at 1:20 he is showing the direction you should travel on the unit circle to get the correct coordinates for the graph. if you move the wrong direction, you aren't getting the correct numbers. you can really tell when you start graphing.
1 Comment
Is there some sort of intuition as to why cos is the x of the unit circle and sin is the y?
Comment

Well, yes, it's all based on how we define the angle that we are interested in when we are using the unit circle (most people call that angle θ, theta). Just as a convention, we define θ as the angle from the X axis, to the line that goes from the origin to the point on the circle we are interested in. So when we talk about the point on the unit circle at 45 degrees, ( or pi/4 radians), that's 45 degrees from the positive X axis.

If we have some point on the unit circle, and we draw a line from the origin to that point, that gives us 1 leg of a triangle. If we use the X axis as the second leg of the triangle, and then draw a line that goes through the point on the circle to the x axis (and we make sure the line is perpendicular to the x axis), that gives us the third leg of our right triangle. If you do this, you'll see that the angle, θ, that I was talking about before is one of the three angles of this right triangle. It is the angle at the point of the triangle that touches the origin. So if we take the cosine of that angle, that is equal to the adjacent side of that triangle divided by the hypotenuse. But notice that the adjacent side of the triangle is just the x coordinate for the original point on the unit circle that we are interested in, and the hypotenuse of the circle is just 1. So the adjacent side over the hypotenuse is just the x coordinate of the point on the circle divided by 1.

Similarly, the sign of the angle θ is the opposite side of the triangle that we drew, divided by the hypotenuse. But this is just the y coordinate of the point on the circle, divided by 1… which is just the y-coordinate of the point on the circle.

If this doesn't make sense, it may help to draw a pair of x and y axes, and draw a circle on the paper centered on the origin with a radius of 1. Then, pick a point on the circle (and I would pick a point in quadrant 1 until you get comfortable with the concepts), and draw the appropriate right triangle.
Comment
At 2:05 about it states that when cos= 1 then theta is 0; I'm confused, I thought cos equalled one at 2π.
1 Vote
Comment
2pi and 0 are the same thing. 2pi is once all the way around the circle.
Comment
Cant the value of theta that give cos(theta) = 1 be :
-π , -3π , -5π and so on ? As cos(theta) also becomes -1 as we go counterclockwise in the unit circle .
So can't it be represented as -2πn + π ?
1 Vote
Comment
Your answer and Sal's answer are actually the same because n can be any integer. This, when n is negative for your expression but positive (with the same absolute value) for Sal's expression, both expressions will have a value of 1 when the cosine is taken of them. However, your answer isn't "uniform" because the multiple of n must be positive and the added constant must be in the interval [0, 2*pi) for the answer to be "uniform."

Example:
2πn+π for n=1 and -2πn+π for n=-1
3π and 3π
Take the cosine of both expressions.
-1 and -1

I hope this helps!
Comment
How do you calculate the cos/sin/tan/other trigonometric functions for imaginary numbers?
Comment
You may not be able to. I don't think theta can be imaginary.
1 Vote
Comment
Doesn't cos=1 at -2pi also?
And also, doesn't the question say just "in the graph below"? Why doesn't he just end the answer with cos(theta)= -2pi, 0, 2pi ?
1 Vote
Comment
Yes, it continues on that pattern toward an infinity of radians, both positive and negative. I cannot speak for why Sal did not stop at the narrow definition of the question, but at that point in the video, he was showing that the pattern continues on and on, and how to find out how it continues. He gave us the general form for all values when cos(theta) =1 and -1, which was also a *bonus result* to help us calculate for ourselves, but was not part of the original question. It is true that on an exam you would probably want to stick to the exact answer as requested.
Comment
Is it ever possible for a graph of COS or SIN to have a value that is undefined
1 Vote
Comment
Sec(x) is undefined when cos(x) is zero, because sec(x) is 1/cos(x). You can't divided by zero. Same thing with cosecant. Tangent is undefined at pi/2 and 3pi/2, because the line through the origin becomes vertical at those points.
Comment
What is Theta?
Comment
It's a greek letter which often represents an angle.Like a variable
Comment
4:40 Couldn't you also say pi*n, for n of all odd integers?
1 Vote
Comment
Yes, but it's a bit clunkier. Sal's solution is more conventional. We tend to like to define n as a member of the integers (a well-known set of numbers) and then write theta=(2n+1)pi, which takes care of the "odd numbered multiples" of pi we're after.
Comment
at 3:50, why is it that it goes pi, 3 pi, 5 pi for cos (-1) rather than o, 2, 4 pi...?
1 Vote
Comment
The cosine is just the x-value of a point on the unit circle. At 0, 2pi, 4pi, etc, the x-value is 1. But at pi, 3pi, 5pi, etc, the x-value is -1.
Comment
What would you do if you are trying to find the zeros of a function, for example 3 cosx?
1 Vote
Comment
The graph of 3 cos(x) has the same zeros as the regular graph of cos(x), despite that 3 cos(x) is vertically stretched by a factor of 3.
1 Vote
Comment
At 0:58 why does Cos π/2 equal to 0?
1 Vote
Comment
π is equal to 180 degrees. Therefore 2π equals 360 degrees, which is along the x axis. So looking at the unit circle Cos is equal to X.
1 Vote
Comment
How would I find the natural domain of trigonometric functions such as: 3/(2-cosx)?
All I'm interested in is the method/concept involved.
1 Vote
Comment
Domain is always the values of "x" where a you get a real solution.
So ask yourself "what makes this function unsolvable?"
The most common culprits are square roots of negatives and dividing by zero (though others do exist). In this case, is there anything that makes the denominator equal to zero? If so, you would say the domain is all x EXCEPT for x="whatever those are."
1 Vote
Comment
Why does Sal use the "y" for the vertical axis for the sinusoidal graph? I think it would be less confusing if Sal plotted "X" for the vertical axis for the sinusoidal function. I think the confusion for me was that you look and at the sinusoidal wave and see "y" when the cosine for a unit circle plots the X-cordinate value. You think you should be looking at the y-co-ordinate axis on the Cartesian co-ordiante graph, but Cos theta = Adjacent/Hypotenuse where the Hypotenuse is "1." Therefore, you should be looking at the X-co-ordinate for the various cosine angles.
1 Vote
Comment
How do I find "b" of a cosine graph? I do not understand the cyles and how to count them.
1 Vote
Comment
B is the coefficient in front of the X value in the parentheses.
To find it you take the period of the graph (the time it takes to repeat itself) and set that equal to 2B. This works because the period in the parent graph [f(x)=Cos(x)] the coefficient in front of your x value (or B) is 1, yet the period is 2. This means the period is always twice as much as your B value.
1 Vote
Comment
when exactly will n be represented in (2pi) n
1 Vote
Comment
I think what you mean by your question is, "Why do they put the n there and what does it mean?"
If not, could you please clarify? Thanks. :)

Meanwhile, I will answer what I believe you are asking.
The n represents all real numbers. These are also known as "all reals" or just as "R".
So it can be a 5, a 50, a billion... whatever real number you please.
So why do we put it there? Well, instead of listing all the infinite multiples of 2pi, we put n and say that n = all reals.
Okay. So what does it mean? It tells us that the answer is any multiple of 2pi. In trig, this means that every time we make a full circle, we have the wanted answer.

Since your asking this question on the "graph of cosine" video, I assume you wanted to know this question due to a cosine problem.
And what n(2pi) means when we are talking about cosine is that every time we make a full circle, we are going to get 0(on a unit circle).

Now I have a question for you: Are you sure it said n(2pi), not n(pi)? Because you get the same answer for cosine in both answers....

Hope this helps. :)
Sylvia.
1 Vote
Comment
How do u know when what part cuts the line and where i have a bad understanding of this part
1 Vote
Comment
No offense, sir, but I have a bad understanding of this question.
1 Vote
Hi, I'm not quite understanding why cos(-pi)=cos(pi). cos(pi) gives me negative one, so shoulden't cos (-pi) give me the reverse of positive one? Thanks so much.
1 Vote
Comment
Well, look at the graph of cosine. It is even, which means that it is symmetric around the y-axis. If something is symmetric is around the y-axis, then f(x)=f(-x). So, cos(-pi)=cos(pi)= -1.
1 Vote
Comment
How would one find the x-intercepts of a trigonometric function given the equation and not the graph?
1 Vote
Comment
Well, basically, if you want to know the x-intercepts, you want to know *when the value of the function is equal to zero*, because the value of f(theta) is the vertical axis in this case.

So, *it depends on the function* from that point.
When does sin(theta) = zero? According to SohCahToa, sine is opposite over hypotenuse, and sine(theta)'s value is zero when the opposite side is zero. After a while you will have this figured out, but until you do, o figure this out, draw a little picture of the unit circle like Sal did at 0:30
Now, when is the opposite side (of the unit circle triangle) equal to zero? It happens twice on the way around the unit circle: when the hypotenuse is squashed against the x-axis. This occurs when theta = 0 and also when theta = pi (180 degrees from the initial side)

But we don't stop there, because the circle is continuous--the angle can keep on increasing, going around and around and around past the horizontal axis every time it passes through another pi's worth of revolution.
So sin(theta) =0 every additional pi, and if you go the other way around the unit circle, it has a value of zero at every additional -1pi revolutions. So for what you need to know, on a graph of the sine function graphed against the angle in radians, the x-intercepts will occur at every positive and negative whole number multiple of pi continuing out *forever*, in addition to when the angle measures 0 radians ( degrees)

Sal just did the cos(theta) above, and the x-intercepts for cosine are staggered from sine by 90 degrees (pi/2 radians). Now we care about Coh, or opposite over adjacent on the unit circle. Again, you will have an x-intercept whenever the value of cosine equals 0.

This happens at pi/2, which is 90 degrees, because that is when the length of the adjacent side of the unit circle triangle becomes zero. It also happens at every additional added 180 degrees beyond pi/2, for example at 3/2 pi radians, 5/2 pi radians (270 degrees and 450 degrees and on and on in the forward direction). If you rotate the angle backwards, the value of cos(theta) is zero as you subtract pi (180 degrees) from pi/2 radians. So cos(theta) =0 and you have x-intercepts every negative rotation that ends with the value of 0: -3/2 pi, -5/2 pi and on and on.

The graph of tangent is strange-looking because it runs off the graph at regular intervals, looking like a row of snakes. Tangent is not defined when the angle is pi/2 because tangent equals opposite over adjacent, and at 90 degrees, the adjacent side is zero. It does have an x-intercept every time sine equals zero and cos equals one (or negative one), which happens at theta = 0 and theta = pi and theta = 2pi and also, going the other direction, at theta = -1pi radians and theta = -2 pi radians.

Cotangent looks like tangent, only the snakes wiggle the other direction and the x-intercepts occur at offset from the intercepts of tangent. Cotangent(theta) equals zero when cosine equals zero, in other word when theta = pi/2, 3/2 pi, 5/2 pi, and also, -pi/2, -3/2 pi and so on.

Well, that is a long answer and I have not gotten to cosecant and secant, but never fear. The process is the same.The surprising result for secant is that it is the inverse function for cosine, and its value *never crosses the x-axis*. Same thing for cosecant, the inverse function for sine. So the quick answer is there are no x-intercepts for secant and cosecant.
1 Vote
Comment
I had it. At least, I thought I had it. I've understood everything up to this video. I cannot understand why, when he starts with theta = 0, and states that at this point the X coord is 1, does he then place the point at Y = 1? It seems counterintuitive to everything I have ever learned about the X and Y axis.
1 Vote
Comment
The unit circle and cosine function are separate graphs. On the unit circle the cosine of any theta is given by the x-coordinate of a point on the circle. But on the graph of the cosine function the value of the cosine is placed on the y-axis instead. That's why he puts the point at y=1 and not x=1. It is a bit confusing at first.
1 Vote
Comment
Do you have a video on how to graph a tangent function? I can't seem to find one on this website. Thanks!
1 Vote
Comment
at 4:44 , can't we just multiply it with and odd integer to find cos thetha=-1?
1 Vote
Comment
as in cos theta there is interval of 2pi for +ve n 2n+1 for -ve is it the same for sin also??
@3:14pm
1 Vote
Comment
How does theta = 0 gives the value of co(theta) = 1 ?
Comment
It is not co(theta) it is cos(theta) cos is the abbreviation for the trignometric ratio cosine. Cos(0) = 1 where theta = 0
Theta 0 means that it is a triangle with a 0 degree triangle like http://math.colgate.edu/~kellen/interspace/0degrees.gif
So the hypotenuse and adjacent both are the same while the altitude is 0. Cos(theta)= adjacent / hypotenuse = 1
what is an asymptote ?
Thank you.
1 Vote
Comment
Hope this can help: http://en.wikipedia.org/wiki/Asymptote (or this definition from Google: a line that continually approaches a given curve but does not meet it at any finite distance)
1 Vote
Comment
"where N is an integ... could probably write that a bit neater" writes it messier.
Comment
At 1:06, he says the x-coordinate is at 0. However, it looks like x is at 1 and the y-coordinate is at 0. Am I missing something here?
Comment
he means to say when x-coordinate is 0 then y=1(pointing that one) and at y-coordinate is 0 x=1
1 Vote
Comment
At 4:15, I'm a little bit confused. Can someone explain? Thanks!
Comment
What does this actually mean scientifically speaking...errr... well
1 Vote
Comment
Discuss the site

Flag inappropriate posts

Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.

abuse
• disrespectful or offensive
• low quality
• not about the video topic
• a homework question
• a duplicate answer
• repeatedly making the same post
wrong category
• a tip or thanks in Questions
• a question in Tips & Thanks
• an answer that should be its own question