Lecture 04: Simple trig identities

September 2, 2015

Start | So, if I told you that sine of v is
3/11 and I gave you either a or c
this would not be very hard.
Because then what you would do is set the ratio. You would say 3 is to 11 as this is to this. In fact we know that the sine of v is is a over c. So, three elevenths...3 is to 11 as a is to c. The problem is that you do not know a and you do not know c. |

0:30 | Right, so that is a bit of an issue.
So, how long, what could we do? We have 2 variables. We could use the Pythagoren Theorem to get another equation. You also know That a squared plus 4 squared equals c squared. So, now we have to do a kind of nightmarish algebra that's the reason we will figure it out. Um, this is the same as a squared plus 16 |

1:01 | equals c squared.
And if you cross multiply, 3 c is 11 a. So, we can do a little substituting, Okay? and solve it. We are trying to find what a is so we want to get rid of c. C is 11 a over 3. Now, we plug it into here. So, a squared |

1:30 | plus 16
is (11 over 3 a) squared.
Alright, lets solve that. A squared plus 16 equals 11 squared, which is 121, a squared over 9. Another board... So, we have... a squared plus 16 is 121 a squared over 9 |

2:03 | We all love fractions.
The best way to deal with a fraction is to multiply everything by 9. Now you get... 9 a squared plus 144 is 121 a squared. You are sitting here saying that I would never do this on an exam. I have faith. Subtract the 9 a squared.. You get 144 is 112 a squared. |

2:31 | Divide it by 112.
Then you take the square root of it. a is the square root of 144 over 112. So, did I make a mistake? Sure. Let's review. We tell you that sine v is 3/11. And, we are trying to find a. So, you go to your triangle and you say that you know that the sine of v is the opposite |

3:01 | over the hypotenuse.
That means that 3/11 is the same as a over c. And, as I said before, the problem is there is 2 variables and we only have one equation. We need another equation. You say, "I don't know another equation." "Except, I love the Pythagorean Theorem. It is the only one I ever memorized." a squared plus 4 squared equals c squared. If a is 3 |

3:31 | right?! and c is 11,
3 squared plus 4 squared is not going to give you 11 squared.
That is the problem. Okay? Um, so, now we have a second equation. Now you just have to do messy algebra. So, the way that you solve this is that you could isolate a or c in this one to get something squared. That is what you want to do. So in this equation, you cross multiply and isolate c and you call it 11 a over 3. And, you place it here for c. Then you square that. |

4:02 | As I said, you multiply through by 9
and you get this nice, not too hard equation.
Subtract the 9. Confused? Another thing that we seem to have this TA versus Professor conflict, is when we give you something like, theta in between Pi over 2 and Pi sin |

4:31 | theta is
4/7
find cosine theta
We did some of these one Monday, but let us do one more.
Okay? There is more than one way to get this right. What you want to do is to find the most straight forward way to get this right. The first thing is where is Pi over 2? Well, Pi is 180 degrees so that is 90 degrees. That is 180 degrees So it is somewhere in the second quadrant. |

5:03 | Remember what I said, "First you need to figure out where to
place the angle.
Now, the sine is 4 over 7. do Pythagorean Theorem of 4 and 7. You could do some other squaring and Pythagorean stuff but there is no need to. We need to find, That side because we need to find the cosine of theta. |

5:30 |
Okay? So, 4 squared
plus b squared is 7 squared.
Okay, 16 plus b squared, is 49. b squared is 33. |

6:01 | b is plus or minus the square root of 33.
We are in the second quadrant so be is in this direction. So, b is going to be a negative value. So, b is going to be negative square root of 33. So, therefore, cosine of theta is SOH CAH TOA. Negative square root of 33 over 7. They are going to capture it on video. I want to keep that on the DL. |

6:31 | Okay. So, some other stuff
you should know.
Sine of a negative angle is negative sine of a positive angle. What does that mean? What does the negative angle mean? Well, this is the angle, theta. Right? You go this way. Negative theta just means go the other way. |

7:00 |
So, that is the corresponding
negative angle. So, if this is
40 degrees
this is negative 40 degrees. If this is
Pi over 5, this is negative Pi over 5.
So, the sine of positive theta is that y.So, the sine of negative theta is going to be negative y. If you know that the sine of |

7:31 | 17 degrees
is point 2.
Then the sine of negative 17 degrees is negative point 2. Okay. Not very hard. The cosine of a negative angle is the same as the cosine of a positive angle. Because they have the same x. So the cosine of this angle is the same as the cosine of the negative angle. Okay? So this is what is called an odd function, |

8:01 | and this is an even function.
Later we will do odd and even functions So, that is one thing you should know. Will that show up in a problem? Well, the problem would be using the sine of a negative angle in it. And we would convert it negative sine of a positive angle. That is all that we would be testing. |

8:30 |
Another thing is this
angle is theta
and since the sum of these two angles is 90 degrees
or Pi over 2.This angle is
Pi over 2 minus theta.
So, I pointed this out last week. The sine of the one angle is the cosine of the other angle. Right, this is A, B, and C. The sine of theta is A over C. Cosine of |

9:00 | Pi over 2 minus theta.
Is also A over C. That tells you that the sine of theta is cosine of Pi over 2 minus theta. Also, known as 90 degrees minus theta. That means that they are complementary angles. 2 angles add up to 90 degrees. or Pi over 2 ok? So, if the sine of the one angle is the cosine of the other angle. |

9:30 | And, of course
cosine of theta
is sine of pi over 2 minus theta.
Suppose that I tell you... sine of 2x-5 equals equals 0. Find the smallest x where that is true. Smallest positive x. |

10:00 |
Well,
for sine of 2x-5 equals
0 sine 0 is 0.
We learned that one. So this just means 2x-5=0. 2x=5 |

10:31 | x=5/2
Cosine of 2x-5=0
Where is cosine equal to 0?
90 also known as Pi over 2. So, when you do these problems, you always do them in radians. So, the cosine of 90 degrees, the cosine of Pi over 2 is 0. So, 2x-5 has to equal Pi over 2. We can just solve that for x. So you add 5. |

11:00 |
and you divide it by 2.
Pi over 2 plus 5 divided by 2. And, you could leave it like that. Okay? You do not need to simplify unless we ask. So, 2x-5 = Pi over 2. So, to solve that you just add 5 to both sides and divide by 2. So, these are all in terms of radians. |

11:32 |
Ah, I said the smallest x.
Smallest positive x where it is true. Now, yes, you could use 3 Pi over 2. You could use 5 Pi over 2. There is an infinite number of answers. So, we want the smallest one. |

12:01 |
How about the smallest positive x where
this is true?
Lets see if you could figure it out. Where is sine equal to 1/2? Lots of places plus not degrees Pi over 6. Do not do it in degrees, you have to do it in radians. So this means the sine of Pi over 6 is 1/2. |

12:31 |
2x-1
will give you Pi over 6.
Now you need to figure out what x is so, add 1 and divide by 2. How about that one? The smallest positive x where that is true. Tangent of 4x |

13:00 | minus 3
is 1.
So, you have to say to yourself, where is tangent equal to 1? Lots of places, but the smallest positive value that is equal to 1 is Pi over 4. Okay? So, now... |

13:30 | you just solve for x, and you add 3.
and divide by 4. You can leave it like that. Do not do not write that. The problem is that it looks like you are writing 3/4. So, if you want to use that slash make sure you put parentheses around the |

14:01 | Pi over 4 plus 3. I want to recommend
not doing it that way.
Okay? So what if I asked you that? What if I was in a really bad mood? So, I said, "How about that one?" It is not as bad as it looks. Because remember just a couple minutes ago, I showed you an identity. |

14:31 | What do you know about the relationship
between the sine of an angle
and cosine? We know that
sine of the one angle is the cosine of
the complementary angle.
So, cosine of x is the same as the sine of Pi over 2 minus x. Okay? So, we could say that the sine of 2x-1 is the same as the sine of Pi over 2 minus x. So, remember, the cosine of the angle is the same as the sine of Pi over 2 |

15:01 | minus the same angle.
Now, I just say 2x-1 is Pi over 2 minus x. Yes? This is x that angle is Pi over 2 minus x. Sine of this angle is the cosine of that angle. The cosine of this angle is the sine of that angle. |

15:31 |
So, sine of x is
B/C
Cosine is Pi over 2
minus x
is also B/C
Therefore the sine of x
is the cosine of Pi over 2 minus x.
That is where that came from before. |

16:00 | And, the cosine of x is the sine of Pi over 2 minus x.
But, if we give you something like this This is your typical Webassign homework meaning that this caliber of problem. If you just replace the cosine with sine of Pi over 2 minus x. The sine of this is the sine of that and you can pull away your sines. So, 2x-1 is just equal to Pi over 2 minus x. Add x to both sides. Add 1 to both sides. x equals Pi over 2 plus 1 all over 3. |

16:30 |
This should take you a minute.
So, if you want to turn this into cosine. I could turn this into sine or I could turn this into cosine. It would be cosine of Pi over 2 minus whatever this angle is. So, this angle |

17:00 | is x minus 2.
Be very careful when you do this it should be the minus theta here. This is the angle, this is the theta. So this is the Pi over 2 minus theta. Okay? You sure? I see a lot of people confused on this. You have to put it in parentheses because it would otherwise mess up your minus sign. Maybe you would not mess up the minus sign, |

17:30 | but most people will.
This means 4x-5 equals Pi over 2 minus (x-2) So, 4x-5 is Pi over 2 minus whatever that angle is. Okay? This becomes 4x-5 equals Pi over 2 minus x plus 2. |

18:00 |
So, 5x
equals
Pi over 2
plus 7.
Because you divide by 5. Okay? x equals Pi over 2 plus 7. All over Pi. Yes. It is the same thing. Yes. Not 90 degrees though. |

18:30 | It must be Pi over 2.
Oh, you also could have made this into sine. Okay? You could have said sine of x minus 2 is the sine of Pi of 2 minus 4x-5. In the end you get the same angle. |

19:00 |
You can convert this to
sine or you can convert this
to cosine. It doesn't matter.
Okay? So, this is going to be the same as sine of Pi over 2 minus 2-x. |

19:30 |
Distribute the minus sign.
Well, you can throw away the sine. And you can say, 3x+ 4 is Pi over 2 minus 2 plus x. You must get that minus sign in there. Pi over 2 minus whatever that angle is. |

20:01 | Subtract x from both sides.
Subtract 4 from both sides. Divide by 2. Okay? Feeling happier? Good. So, let us see, now, what these would be. Sine of theta plus Pi. Cosine of theta plus Pi. |

20:31 |
What does that mean to be plus Pi?
Well, let us think about the unit circle for a second. If that is theta. If you are going Pi it is going 180 degrees. So that is coming around to the other side. This would be theta plus Pi. I went Pi. I went half way around the circle. So, sine here is the negative sine |

21:01 | here. The cosine
here is the negative cosine
here.
So, by doing theta plus Pi I am switching the signs from positive to negative or from negative to positive. So, again this angle theta. When I went this far. Then, adding another Pi means I am going half way around the circle. I am doing 180 degrees. That means now I am down here. So, all that happens is values |

21:31 | that were positive here, become negative here.
and values that become positive here were negative here. And, the other way around. And, of course if I add 2 Pi, I get back where I started. |

22:01 |
So, what does 2 Pi mean? It means that I went all the way around the circle.
In fact when we have 2 Pi, we are back where we started and everything starts repeating. So that is called a period. A circle is one full period and it starts over. |

22:30 |
So, where is theta plus 2 Pi?
Well, that is theta, that is theta plus 2 Pi. I went one full cycle around. In fact, anytime I go Pi. I change the signs from positive to negative and negative to positive. Anytime I go 2 Pi, I get back where I started, so, |

23:00 | If I take the
sine of theta and I
add k time Pi.
Where k is an integer. So, I have sine of theta plus k Pi k is an integer. Then from that you pull, Sine is theta if k is even or negative sine |

23:30 | of theta
if k is odd.
Similarly, If I have cosine of theta plus k pi that will be cosine of theta when theta is even. |

24:00 | negative cosine of theta
if k is odd.
So, this helps us figure out something like. sine of 13 Pi over 6. One thing you need to do of course is to turn that into degrees. Another thing you can do of course is say |

24:31 | that is Pi over 6
plus 12 Pi over 6.
Why do I need the 12 Pi over 6? Because that is 2 Pi. That is the same thing as sine of Pi over 6 Plus, 2 Pi. And we know that if i add 2 Pi, |

25:00 | and because 2 is an even number
that is just the same
as sine of Pi over 6.
Did we all memorize our charts yet? NO?! You should get them tattooed. Tattoo it right about here. So, you can cross your leg in the test. You can look down and "Oh!" It is equal to a half. |

25:30 |
If I give you this, you should be able to convert it to degrees.
We should try to figure out how many 2 Pis am I adding to this? That is another way to figure it out. So, one thing you can do to make your life easier, just sort of start subtracting 2 Pis Why 2 Pis? Anytime I add 2 Pi I go once around the circle. Okay? So if I say, how many times around the circle do I go? Well, let's subtract 2 Pi. |

26:00 |
That is 17 Pi over 4 minus
8 Pi over 4.
Which is 9 Pi over 4. Okay? So, sure. 17 Pi over 4 is where? Well, I have to start somewhere and it is bigger than 2 Pi, so I must have gone around the circle more than once. The question is how far did I go? I do not know. You just take the 2 Pi away |

26:30 | and see where I am. It is like subtracting
360 degrees.
Absolutely! Yes. Okay. Let us subtract another 2 Pi. Which is 8 Pi over 4. Which just leaves Pi over 4. So, that means that cosine of 17 Pi over 4, is going to be the same thing as cosine of Pi over 4. |

27:01 |
Because 17 Pi over 4
is really Pi over 4
plus 4 Pi.
What is the cosine of Pi over 4? How did I get the 8 Pi over 4? 2 Pi is 8 Pi over 4. 2 is 8 over 4. |

27:31 |
It could mean 12 Pi over 6, but I want
to have Pi over 4 when I am done.
How about |

28:01 | 41 Pi over 6
41 over 6 is bigger than 2 right?
So, it is 41 Pi over 6 is bigger than 2 Pi. So, I have to figure out what angle this is. It is less than 2 Pi. Everybody agree? It gets down to some angle less than 2 Pi. So, 41 Pi over 6 I will subtract 2 Pis. So, 2 Pi is 12 Pi over 6. I am just doing fraction work. |

28:30 | I am going to take 41 6ths and I am going
to subtract
12 Pi over 6.
and I get 29 Pi over 6. Is 29 over 6 bigger than 2? Yes! So I need to take away 12 over 6 again. Is 17 over 6 bigger than 2? |

29:00 | Yes! I need to take away another 12 over 6.
I could have done it all in one step of course. Now I am down to 5 Pi over 6. 5 Pi over 6 is not. Now I know where to stop. Cause now I am working with a number that is less than 12 Pi over 6. Less than 2 Pi. Which is 12 over 6. So, sine of 41 Pi over 6 is the same as saying, sine of 5 Pi over 6. |

29:31 | So, then, we need to figure out what
is the sine of 5 Pi over 6.
150 degrees? You do not have to convert it to degrees. But, most of you are more comfortable with that. So, 150 degrees is about there. Our reference angle is 30 degrees. So, this is the same as the sine of 30 degrees or the sine of Pi over 6. |

30:00 | The sine of 30 degrees is...
1/2.
Now is it positive or negative? Positive because we are in the second quadrant. So, therefore, the sine of 41 Pi over 6 is 1/2. |

30:30 | By the way, we could always turn this into degrees and subtract 360. It is |

31:00 | exactly the same thing.
You can turn it into degrees subtract 360 or you can leave it in radians and subtract 2 Pi. They are the same thing. We got to subtract 2 Pis. Alright, now, you can do it all at once, by the way. You could look at this and say, 8 Pi over 4 is 2 Pi. So, you look at this and say, okay now you know I will need more than one 8. You may not notice that I can subtract 16 right away, |

31:30 | but you will.
So, 23 Pi over 4 minus 2 Pi is 23 Pi over 4 minus 8 over 4 is 15 Pi over 4. Take away an 8 over 4 again. |

32:00 |
Now I have 7 Pi over 4.
7 Pi over 4 is less than 2 Pi. So this is the same as the cosine of 7 Pi over 4.Let's not convert this into degrees. Let us do it in radians. All the way around, is 8 fourths. So this is one fourth less than that. So, 1 Pi over 4 |

32:30 | less than
M 8 Pi over 4.
So, that is going to be Pi over 4. Okay? You understand that? It is 8 fourths all the way around. So, 7 fourths we have 1 fourth left over. So, this is the same as the cosine of Pi over 4 which is Why is it not 3 Pi over 4? |

33:01 | Because, once I subtracted
the 2 Pis I got down to 7 Pi over 4
Okay, and that is where I stopped because that number
is less than 2 Pi.
Well, it is okay to say negative Pi over 4. Yes. Is it positive or negative? Good. Okay. So, where did I get 8 fourths from? I am doing |

33:31 | fractions. Okay? That is 23 fourths.
And I need to take away 2. So, 23 fourths minus 2 is 23 fourths minus 8 fourths is 15 fourths. Okay? 15 fourths minus 2. Is 15 fourths minus 8 fourths. 7 fourths. So, I covered almost everything that I wanted to cover today. But, not everything. So, Wednesday we are going to do graphs. |