You are Here: Home >< Maths

# C3 June 2005 Question watch

1. I've done part a of question 1.

I've nearly done part b of question 1.

I've got to the stage:

2secθ = - 3

secθ = -3/2

cosθ = -2/3

θ = 131.8

In the four quadrants, where would 131.8 go since θ is positive, but it's for cos...and the angle 131.8 is obtuse?

How would I calculate the 2nd angle?

Thanks
2. For solutions I just sketch the graph of y=cos(x)
The answer I get from that is 228.2..
3. (Original post by Kneechuh)
For solutions I just sketch the graph of y=cos(x)
The answer I get from that is 228.2..
Thanks for your reply. I'm not too familiar with using the graph sketching approach, please could you show me with the CAST diagram if you know?

Thank you again!
4. (Original post by gottastudy)
Thanks for your reply. I'm not too familiar with using the graph sketching approach, please could you show me with the CAST diagram if you know?

Thank you again!
I don't really get the CAST diagram thing, sorry.

Like I said, I sketch y=cos(x), and then draw a horizontal line roughly where my principle value is (in your case at -3/4), and then look at where it intersects the cos(x) curve. In our case, the line cuts the curve twice, with our first point being at 131.8, and the second point being at 228.2 (180 + (180-131.8)). We can get this as due to the symmetry of the cos(x) curve, it's the same distance away in either direction. I MSPaint'd an awful representation :

I probably explained it badly, so don't feel silly if you don't understand...

To get your principal angle, you find arccos of 2/3 and not negative 2/3. The negative sign is there to tell you which quadrant the angle is in.
6. (Original post by Kneechuh)
I don't really get the CAST diagram thing, sorry.

Like I said, I sketch y=cos(x), and then draw a horizontal line roughly where my principle value is (in your case at -3/4), and then look at where it intersects the cos(x) curve. In our case, the line cuts the curve twice, with our first point being at 131.8, and the second point being at 228.2 (180 + (180-131.8)). We can get this as due to the symmetry of the cos(x) curve, it's the same distance away in either direction. I MSPaint'd an awful representation :

I probably explained it badly, so don't feel silly if you don't understand...
Thank you for taking the time to draw that! No, I do get your method now. I did sketch y=cos(x) and got two points at the bottom but was unsure about the symmetrical thing. Now I get it.

(Original post by Doughboy)

To get your principal angle, you find arccos of 2/3 and not negative 2/3. The negative sign is there to tell you which quadrant the angle is in.
Thank you! Is my sketch correct?

7. (Original post by gottastudy)
Thank you for taking the time to draw that! No, I do get your method now. I did sketch y=cos(x) and got two points at the bottom but was unsure about the symmetrical thing. Now I get it.

Thank you! Is my sketch correct?

I've uploaded some C3 documents. They address the exam language, common mistakes, 'hot tips' and glossary of C3 terms.
Attached Images
8. C3 Hot Tips.pdf (135.2 KB, 128 views)
9. C3 Glossary.pdf (175.4 KB, 101 views)
10. C3 Exam Language.pdf (153.0 KB, 105 views)
11. C3 Common Mistakes.pdf (169.7 KB, 196 views)
12. (Original post by Doughboy)

I've uploaded some C3 documents. They address the exam language, common mistakes, 'hot tips' and glossary of C3 terms.
Thank you for checking my diagram and thank you so much for uploading those! I'm printing them out now to read. I really appreciate it, rep for you after 00.00

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 17, 2010
The home of Results and Clearing

### 3,234

people online now

### 1,567,000

students helped last year
Today on TSR

Hang on, have Edexcel's come out already?

### University open days

1. SAE Institute
Animation, Audio, Film, Games, Music, Business, Web Further education
Thu, 16 Aug '18
2. Bournemouth University
Fri, 17 Aug '18
3. University of Bolton
Fri, 17 Aug '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams