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### Course: Algebra 2>Unit 11

Lesson 3: The Pythagorean identity

# Using the Pythagorean trig identity

The Pythagorean identity tells us that no matter what the value of θ is, sin²θ+cos²θ is equal to 1. This follows from the Pythagorean theorem, which is why it's called the Pythagorean identity! We can use this identity to solve various problems. Created by Sal Khan.

## Want to join the conversation?

• Why is it written sin^2(θ) + cos^2(θ) = 1 instead of sin(θ)^2 + cos(θ)^2 = 1? The second way seems to better represent how the math is done, so why is it notated the first way?
• Because when you put an exponent on a set of parentheses, it means to raise the value inside the parentheses to the power. Hence, sin(θ)^2 means "take the value of θ, square it, and THEN find the value of the sine function." which is very different from sin^2(θ) which means "find the value of the sine function for θ and then square the result". Note that sin^2(θ) and [sin(θ)]^2 are equivalent expressions. Also, sin(θ^2) and sin(θ)^2 are equivalent expressions.
• Sal said in that that point is clearly a negative value, I don't understand how this works. Can somebody explain to me or tell me where I can learn about this?
• Any of the unit circle lessons really. You just need to get an intuition for when certain angles cross over the axes.

Hopefully you understand that an angle of 0 lies ont he positive x axis. On the unit circle specifically it is point (1,0) so it's positive x and 0 y

the moment you give it an angle at all x stays positive, and now y is positive too. This is because up until 90 degrees (or pi/2 radians) the circle is in quadrant 1

at the right angle when it reaches the y axis y is still positive, but now x is 0

quadrant 2 has x negative now, since it is on the left of the y axis. if it's easier you can remember x = 1 is on the right of the y axis, and x = -1 is on the left.

at 180 degres, or pi radians we are at the x axis again, but on the negative side. again, y is 0 but x is negative rather than an angle of 0.

third quadrant is the quadrant on the negative x axis side and negative y axis side, so both x and y are negative here.

270 degrees/ 3pi/2 radians is the negative y axis, so x is 0 and y is negative. for the axes specifically, if it helps to remember if you are ON an axis then the non axis coordinate is 0. So here the y axis has the y coordinate negative and the x coordinate 0.

Finally quadrant 3 has the x axis positive and y axis negative, then after this it is back to the positive x axis.

Does that help at all? In the video the question starts with saying the angle is -3pi/2 and -pi. Since they are negative they are measured counterclockwise, were -pi/2 is the negative y axis rather than the positive one. So this means -3pi/2 is the positive y anxis and -pi is the negative x axis. the space between these two points is quadrant 2, which as I described above has a negative x value and positive y.
• At , why does the square root of 3/4 become the square root of 3 over 2?
• Because 4 is a perfect square and becomes 2 when you take its square root, while 3 is not a perfect square, so it stays root 3 as the numerator while root 4 is simplified to 2.
• The assumption of x = cos θ and y = sin θ is valid as long as it is a unit circle including the pythagorean trig identity of cos^2 θ + sin^2 θ = 1. In the above problem, it is not mentioned that we are dealing with unit circle.

My question - Can the pythagorean trig identity be used for any circle ?
• (𝑟 cos 𝜃, 𝑟 sin 𝜃) are the coordinates for a point on a circle with radius 𝑟.

cos²𝜃 + sin²𝜃 = 1 is true for any circle.
• why would we change 1/2 to 1/4 at about
• 1/2 times 1/2 = 1/4 at he mentions why he puts it there and then you must solve it out. So it equals 1/4
• At about Sal mentions that the tangent of an angle is equal to the slope of the terminal side of the angle (if you think of the terminal side as a line). What is the reason for this?
• slope is rise over run, or y/x. tangent is sine over cosine, or y over x, at least when dealing with the unit circle. But either way you get the same ratio.

If you don't get why sine over cosine is y/x remember sine is opposite over hypotenuse, and the opposite side when measuring the angle inside a unit circle, is always the y coordinate, and hypotenuse is always 1. Same reasoning for why cosine is x.

Then you need to remember slope can be scaled. if the slope is 3/2, that's the same as saying 6/4, or anything else that simplifies to 2/3, so you can scale it so it makes a right triangle with a hypotenuse of 1.

Also, it might even be simpler to remember tangent is opposite over adjacent, and opposite is y and adjacent it x.

Hope this helped.
• I don't know when is it negative or positive in the "use the Pythagorean identity" assignment, in the video it showed that -3pi/2 < O< -pi to know if its going clockwise or counterclockwise but in the assignment or the work it doesn't show anything can someone explain to me how to figure if its positive or negative
• In the "Using the Pythagorean trig identity" assignment, it indeed does not tell you if it's going to be clockwise or counterclockwise. However, it tells you which Quadrant the angle(theta 1) is located in. Cos(theta) is the x coordinate and Sin(theta) is the y coordinate in (x,y). You just need to think about if the x coordinate (or y coordinate) is positive or negative in that certain Quadrant and which Quadrant (theta 1) is located in.

If (theta 1) is located:
In Quadrant 1, then cos(theta) and sin(theta) are both Positive.
In Quadrant 2, then cos(theta) is Negative while sin(theta) is Positive.
In Quadrant 3, then cos(theta) and sin(theta) are both Negative.
In Quadrant 4, then cos(theta) is Positive while sin(theta) is Negative.

For instance: The angle(theta 1) is located in Quadrant 1, and sin(theta 1)= 11/61.
What is the value of cos(theta 1)?

1. use the pythagorean identity
(11/61)^2+cos^2(theta 1)= 1
cos^2(theta 1)= 1-(11/61)^2
(11/61)^2= 121/3721
1--->3721/3721
2. 3721/3721- 121/3721= 3600/3721
3. cos(theta 1)= square root of 3600/3721
= plus or minus sign 60/61
(-60*-60=3600, 60*60=3600, in this logic which is why the sqrt of 3600/3721 can be positive or negative).
4. To find out if cos(theta 1) is positive or negative, we can see from the problem that it tells us that (theta 1) is located in Quadrant 1, which means cos(theta 1) is positive.
5. Thus, the final answer is cos(theta 1)= 60/61
• How can one input the square root of a number for the exercises?
• You type, "sqrt" and use parentheses () around the number you're taking the root of.
• In the actual problems, I can't figure out how to determine if the answer should be positive or negative with the quadrants because it isn't constant. For example, I have had separate problems where an angle in Quadrant IV was positive in one problem but an angle in Quadrant IV was negative in another problem. The hints don't explain how this is determined and I couldn't find a pattern. (Any pattern that began to form was broken pretty quickly.) Does anybody know how to use quadrants to determine the sign of the answer?
• Whether a trigonometric function is positive is indicated by ASTC

A- All are positive in the first quadrant

S- Sine is the only positive trigonometric function in the second quadrant

T-Tangent is the only positive trigonometric function in the third quadrant

C- Cosine is the only positive trigonometric function in the 4 quadrant.

So the question you mention the trigonometric was positive in 4th quadrant was cosine. While ones that were negative were sin and tan.

Alternatively know that cos theta = x, sin theta = y,
tan theta = y/x and then you could figure out where each trigonometric function is positive based on y and x values in the unit circle. So in the 4 quadrant since x is positive cosine is positive since cos theta = x. On the other hand sin theta is negative because the y value is negative in the 4th quadrant. And hence tan theta is negative since tan theta = y/x