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

    0
    ReputationRep:
    Can someone please help on the following:

    1 i) Express sin2x + cos(90-2x) in terms of a single trig function.

    ii) Find all values of x in the interval 0<x<360 for which sin2x+cos(90-2x) = -1


    2) Find the least positive value of angle A for which sin A = 0.2 and cos A is negative.


    -----------------------

    The first one I'm not sure how to do it now that they're are both sin and cos involved :confused:

    Second one, I'm not sure how to go about doing it.
    • PS Helper
    Offline

    14
    For Q1 have you tried using the angle-addition formula for \cos(A+B)? Then you use your answer to do part (ii).

    For Q2, I assume you mean "least positive" rather than "least possible"? You'll find that there are infinitely many values of A&gt;0 for which \sin A = 0.2, but for some of these, \cos A will be positive, and for the others it will be negative. You need to find the first one such that \cos A&lt;0.
    Offline

    2
    ReputationRep:
    RIP
    • Thread Starter
    Offline

    0
    ReputationRep:
    1)I dont think I've done that angle addition formula thing yet .

    2) I'll try doing it soon.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Rahul.S)
    RIP
    :confused:
    Offline

    2
    ReputationRep:
    What exam board are you on WildBerry?

    I'm on edexcel and haven't come across any questions like these.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Thrug)
    What exam board are you on WildBerry?

    I'm on edexcel and haven't come across any questions like these.
    I'm on OCR, they're from the Misc exercise of their textbook, the topics are usually harder than normal questions .
    Offline

    0
    ReputationRep:
    Wow.. My C2 wasn't as hard as this
    • PS Helper
    Offline

    14
    (Original post by WildBerry)
    1)I dont think I've done that angle addition formula thing yet .

    2) I'll try doing it soon.
    Thinking about it, angle-addition formulae don't come up until C3. In that case, draw a graph of y=\cos (90-2x) and y=\sin 2x on the same axes, and work out what graph transformations are necessary to turn the sine graph into the cosine graph. Then using your C1 knowledge of graph transformations, you'll be able to write \cos (90-2x) in terms of \sin 2x.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by xPixie)
    Wow.. My C2 wasn't as hard as this
    I've been told to do around 15 questions like these

    I've done 10, but there are around 5 more really hard ones that I just dont know how to start. I'll probably just do what I can after doing these, and get the usual C grade written on for not attempting the rest, even though the crap teacher hasn't taught any of this yet. :rolleyes:
    Offline

    0
    ReputationRep:
    (Original post by WildBerry)
    I've been told to do around 15 questions like these

    I've done 10, but there are around 5 more really hard ones that I just dont know how to start. I'll probably just do what I can after doing these, and get the usual C grade written on for not attempting the rest, even though the crap teacher hasn't taught any of this yet. :rolleyes:
    Q1i sounds like a C4 question & I've just started that now
    What board are you? Because I'm on OCR MEI, and their papers aren't as hard as this. If you want to revise, just do past papers, that's the best way to revise maths
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by xPixie)
    Q1i sounds like a C4 question & I've just started that now
    What board are you? Because I'm on OCR MEI, and their papers aren't as hard as this. If you want to revise, just do past papers, that's the best way to revise maths

    Normal OCR .

    I looked at a couple of past papers for these questions, and they're so confusing :confused: Eg the interval is in terms of Pie . Eg 9 : http://www.ocr.org.uk/download/pp_08...l_gce_4722.pdf
    Offline

    0
    ReputationRep:
    (Original post by WildBerry)
    Normal OCR .

    I looked at a couple of past papers for these questions, and they're so confusing :confused: Eg the interval is in terms of Pie . Eg 9 : http://www.ocr.org.uk/download/pp_08...l_gce_4722.pdf
    I have the same questions :P
    Offline

    2
    ReputationRep:
    Ah well I can do intervals in terms of pi, but not what you have mentioned above.
    Offline

    2
    ReputationRep:
    (Original post by WildBerry)
    :confused:
    use sin and cos to derive to tan. 4rm there its easy!

    RIP WildBerry
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What newspaper do you read/prefer?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

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