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### Course: Get ready for Precalculus>Unit 4

Lesson 4: Trigonometric values of special angles

# Trig values of π/4

Sal finds the trigonometric values of π/4 using the unit-circle definition. Created by Sal Khan.

## Want to join the conversation?

• I dont really get the questions in the Trig Values of Special angles activity, cound anyone explain?
(75 votes)
• It actually requires more knowledge than Sal has taught us to this point. It's very strange how they've not taught us this in Algebra and say not to use a calculator. I found this link which is very useful https://www.mathsisfun.com/geometry/unit-circle.html
(150 votes)
• I'm stuck on a problem on the special angles because I don't know how to make a squareroot sign.
(38 votes)
• I believe sqrt(x) would be acceptable on khanacademy.
(140 votes)
• At , Why would the sqrt of 1/2 be 1/sqrt2 and not sqrt 1/2?
(37 votes)
• This is because the sqrt(1/2) is the same thing as saying sqrt(1)/sqrt(2) and since the square root of 1 is just 1 we can simplify the numerator and rewrite it as 1/sqrt(2).
(108 votes)
• Can anyone show how we know that sin(pi/6)=1/2 ? Or from the sin=1/2, how do we get the angle pi/6 ? His explanation for the angle pi/4 makes perfect sense, but do we just have to accept that sin(pi/6)=1/2 and cos(pi/6)=sqrt3/2 ?

It would be nice if someone would at least say: "It was found using extremely difficult and time consuming math that is too much trouble to explain." and then maybe point towards a resource to check it out.

Update:

I figured it out! What nobody here has been saying, because it must have seemed so obvious, is that the angle pi/6 is half of an equilateral triangle! All three angles are 60 degrees (pi/3). Cut it into two right triangles and you get an angle of 30 degrees (pi/6). That also means that the opposite side is going to be exactly half of the hypotenuse. In a unit circle that means that sin=1/2. From there we can work out cos=sqrt3/2

I might be dumb for not seeing the obvious, but there really ought to be a video for this.
(60 votes)
• You're not dumb! You figured out an important principle in trigonometry involving the square root of an irrational number. You're also persistent in that you didn't just sigh and quit, you worked at it until a solution became clear. Good for you, Amos . :-)
(42 votes)
• At , how is the tangent equal to the slope of the hypotenuse? Is it always the case?
(19 votes)
• If you look at the soh cah toa definition of the tan function, we see that the tan is the opposite divided by the adjacent. This is the exact definition of a slope: the amount of rise (opposite) for distance traveled (adjacent).
(43 votes)
• Could you also please help with the exercises because they don't look too much related to me.
(11 votes)
• (24 votes)
• for all those stuck in algebra 2 of khan academy and are confused with the next exercise (Trig values of special angles) you need to go watch https://www.khanacademy.org/math/trigonometry/unit-circle-trig-func/xfefa5515:special-trigonometric-values-in-the-first-quadrant/v/cosine-sine-and-tangent-of-6-and-3
(17 votes)
• At , Sal finds the side lengths of the isosceles triangle, and that is easy to understand. If the two side lengths are different, how would you find the lengths?
(17 votes)
• It's still SOH CAH TOA, but you might have trouble getting your calculator to spit out the right answer unless it's set in radians. Eg if your calculator is set in degrees then Cos(pi/3)=0.86 but if set to radians it'll be 0.5 which actually works.
(0 votes)
• what does this video have anything to do with the practice exercises
ill greatly appreciate any help
i dont understand how to do the exercises
thanks in advance
(11 votes)
• You are correct Unit 11 lesson 4 is incomplete.

If you need find any other resources you always

a) Google "Special triangles"
b) Use YouTube
c) Use a textbook
d) Search through the khan academy Q/A

I will mention this in feedback to khan academy

Edit: It seems like the videos on the trigonometry section are complete but not algebra 2
(11 votes)
• can we take pie as 180 in trig functions so that it will easy to term angles in some problems?
(7 votes)
• Yes, π radians is equal to 180 degrees. You have to make sure that you understand what you are doing though. If you put in an input in degrees, your answer will be in degrees. If you put in an input in radians, your answer will be in radians.
(13 votes)

## Video transcript

Voiceover:So we have depicted here is a unit circle centered at point A and the point B lies on the circle. And then we dropped a perpendicular from point B to point D, point D lies on the positive X-axis. So they form this triangle ABD, and they tell us at angle BAD, angle bad, it has a measure of pi over four radians. What I want to do, in this video, is use our knowledge of trigonometry and use our knowledge of triangles in order to figure out several things. So the first thing we want to figure out is what's the radian measure of angle ABD? Actually let's just do that first and then I'll talk about the other things. that we need to think about. So I assumed you've paused the video and tried to do this on your own. So let's think about what ABD would be. We know that two of the angles of this triangle, so if you know two of the angles of this triangle, you should be able to figure out the third. Now what might be a little bit unfamiliar, we're use to saying that the sum of the interior angles of a triangle add up to 180 degrees, but now we're thinking in terms of radians. So we could say that the sum of the angles of a triangle add up to, instead of saying 180 degrees, 180 degrees is the same thing as pi radians. So this angle plus that angle are going to add up to pi. So lets just say that this right over here, lets just say measure of angle ABD in radians, plus pi over four, plus, this is a right angle. What would that be in radians? Well a right angle in radians, a 90 degree angle in radians, is pi over two radians. So plus pi over two. When you take the sum of them, the interior angles of this triangle, they're going to add up to pi radians. Which is of course the same thing as 180 degrees. And now we can solve for the measure of angle ABD. Measure of angle ABD is equal to pi minus pi over two, minus pi over four. I just subtracted these two from both the sides. So this is going to be equal to, you can put a common denominator of four, then this is four pi over four, this is minus two pi over four, and this, of course, is minus pi over four. So four minus two minus one is going to get us to one. So this is going to get us to pi over four. So the measure of angle ABD is actually the same as the measure of angle BAD, it is pi over four. So that angle right over there is pi over four. Now what does that help us with? So if we know that this is pi over four and that is pi over four radians, and once again we know that this is a unit circle. So we know the length of segment AB, which is a radius of the circle or is the radius of the circle is length one, what else do we know about this triangle? Can we figure out the lengths of segment AD and the length of segment DB? Well sure, because we have two base angles that have the same measure, that means that the corresponding sides are also going to have the same measure. That means that this side is going to be congruent to that side. I can reorient it in a way that might make it a little easier to realize. If we were to flip it over, maybe not completely fit it over but if we were to make it look like this. So the triangle, we could make it look like, a little bit more like this. Actually I want to make it look like a right angle though. So my triangle, let me make it look like, there you go. So if this is D, this is B, this is A, this is our right angle. Now this is pi over four radians and this is also pi over four radians. When your two base angles are the same, you know you're dealing with an isosceles triangle. Because they're not all the same, it's not equilateral. If all the angles were the same this would be equilateral. This is an isosceles non-equilateral triangle. So if your base angles are the same, then you also know that the corresponding sides are going to be the same. These two sides are the same, this is an isosceles triangle. So how does that help us figure out the lengths of the sides? Well if you say that this side has length X, that means that this side has length X. If this side has length X, then this side has length X. and now we can use the Pythagorean Theorem. We can say that this X squared plus this X squared is equal to the hypotenuse square is equal to one square. Or we could write that two X squared is equal to one or that X squared is equal to one over two or taking the principal root of both sides, we get X is equal to one over the square root of two. And a lot of folks don't like having a radical denominator, they don't like having a rational number in the denominator. So we could rationalize the denominator by multiplying by the square root of two over the square root of two, which would be, let's see the numerator will have the square root of two, and the denominator we're just going to have square root of two times square root of two is just two. So we've already been able to figure out several interesting things We've been able to figure the angle of ABD in radians. We're able to figure out lengths of segment AD and the length of segment BD. Now what I want to do is figure out what are the sine, cosine, and tangent of pi over four radians, given all of the work we have done? So let's first think about, what is the sine? We'll do this in the color orange. Given all the work we have done, what is the sine of pi over four radians? And I encourage you once again, pause the video, think about the unit circle definition of trig functions, and think about what this is. Well the unit circle definition of trig functions in this angle, this pi over four radians, is forming an angle with the positive X axis. And where it's terminal ray intersects the unit circle, the X and Y coordinates of this point are what specify the cosine and sine. So the coordinates of this point are going to be the cosine of pi over four radians is the X coordinate and the sine of pi over four radians is the Y coordinates, sine of pi over four. So what's the Y coordinate going to be if we want pi over four? Well it's going to be this length, right over here. Which is the same thing as this length, which is the length of X, which is the square root of two over two. Now what is the cosine of pi over four? Once again I encourage you to pause the video to think about it. What's this X coordinate? The X coordinate, is this distance right over here, which is once again going to be X, which is square root of two over two. Now, what is the tangent of pi over four going to be? Well the tangent of pi over four is sine of pi over four over cosine of pi over four. Now both of these are the exact same thing. They're both square root of two over two. So you're going to have square root of two over two, divided by two over two, well that's just going to give you one. And that also makes since, because remember the tangent of this angle is the slope of this line. And we see the slope, for every X we move in the horizontal direction we move X up. So our change in Y over change in X is essentially X over X, which is equal to one.