Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    0
    ReputationRep:
    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
    Offline

    1
    ReputationRep:
    For solutions I just sketch the graph of y=cos(x)
    The answer I get from that is 228.2..
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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!
    Offline

    1
    ReputationRep:
    (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 :o: :


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

    1
    ReputationRep:
    Well, cosθ is negative is the second and third quadrants. Meaning, to get your first angle, subtract your principal angle from 180 (second quad.) and then add your principal angle to 180 (third quad.)

    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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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 :o: :


    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)
    Well, cosθ is negative is the second and third quadrants. Meaning, to get your first angle, subtract your principal angle from 180 (second quad.) and then add your principal angle to 180 (third quad.)

    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?

    Offline

    1
    ReputationRep:
    (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?

    :yes:

    I've uploaded some C3 documents. They address the exam language, common mistakes, 'hot tips' and glossary of C3 terms.
    Attached Images
  1. File Type: pdf C3 Hot Tips.pdf (135.2 KB, 118 views)
  2. File Type: pdf C3 Glossary.pdf (175.4 KB, 86 views)
  3. File Type: pdf C3 Exam Language.pdf (153.0 KB, 96 views)
  4. File Type: pdf C3 Common Mistakes.pdf (169.7 KB, 187 views)
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Doughboy)
    :yes:

    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 :yes:
 
 
 
Poll
Who is your favourite TV detective?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.