Turn on thread page Beta
    • Thread Starter
    Offline

    1
    ReputationRep:
    If I'm sketching the line rcos t = 2,

    is this just a straight line going in the +ve x direction by 2?

    i.e


    O _______________> l
    2
    Offline

    15
    ReputationRep:
    no man, it's vertical.

    rcost = x, rsint=y
    • Thread Starter
    Offline

    1
    ReputationRep:
    okay - silly error on my part, but then is it just a straight line upwards by 2?
    Offline

    15
    ReputationRep:
    what do you mean 'by 2'.

    in cartesian coordinates, it's the line x=2.
    • Thread Starter
    Offline

    1
    ReputationRep:
    Okay, the question says:

    Sketch the line (or half0line) with polar equation:
    rcos t = 2

    So am I meant to be sketching them on a cartesian or polar graph? I presumed polar.

    In the last question which said sketch t = -PI/3, I simply drew an acute angle of 60 degrees going down the page - was this not correct?
    • Wiki Support Team
    Offline

    14
    ReputationRep:
    Wiki Support Team
    (Original post by studienka)
    Okay, the question says:

    Sketch the line (or half0line) with polar equation:
    rcos t = 2

    So am I meant to be sketching them on a cartesian or polar graph? I presumed polar.

    In the last question which said sketch t = -PI/3, I simply drew an acute angle of 60 degrees going down the page - was this not correct?
    Of course you're meant to be sketching them on a polar graph, but "a straight line upwards by 2" doesn't mean anything. It's the straight line equivalent to the line x=2 in cartesian coordinates.
    • Thread Starter
    Offline

    1
    ReputationRep:
    hmmm - I thought I had this, now I'm getting a bit lost.

    If I'm drawing the polar version of the cartesian eq x = 2, isn't that just this:
    Attached Images
  1. File Type: pdf img128.pdf (3.5 KB, 95 views)
    Offline

    15
    ReputationRep:
    if it were, what would rcost = 3 look like?
    • Thread Starter
    Offline

    1
    ReputationRep:
    ok - then i've not got this yet, can i have a bit of help with this
    • Thread Starter
    Offline

    1
    ReputationRep:
    a bit longer?????
    Offline

    15
    ReputationRep:
    why? why oh why would the value of r be limited to a maximum of 3? and why would the value of t always be pi/2?
    • Thread Starter
    Offline

    1
    ReputationRep:
    Fair enough point, but then I'm lost now
    Offline

    15
    ReputationRep:
    what is r when t=0?

    what about for other values of t?
    • Thread Starter
    Offline

    1
    ReputationRep:
    Okay - I did this, and while it's a rough sketch, is it these pair of parabola's?
    Attached Images
  2. File Type: pdf img129.pdf (4.7 KB, 64 views)
    • Wiki Support Team
    Offline

    14
    ReputationRep:
    Wiki Support Team
    (Original post by studienka)
    Okay - I did this, and while it's a rough sketch, is it these pair of parabola's?


    I'm quite sure we've told you what it is. It's a line. Specifically, it's the cartesian line x = 2. If r cos theta = 2 - imagine it - then drawing a right angled triangle between the pole, the initial line and any point on this line should make it obvious. r cos theta = 2 means the 'adjacent' side will always be 2. That is, the horizontal distance from the pole will always be 2.
    • Wiki Support Team
    Offline

    14
    ReputationRep:
    Wiki Support Team
    See attached diagram.

    The horizontal distance from the pole is fixed at 2. So it must be a vertical line passing through the point (2, 0) (in polar coordinates and in cartesian coordinates). So r cos theta = 2 is equivalent to x = 2.
    Attached Images
     
    • Thread Starter
    Offline

    1
    ReputationRep:
    I follow the triangle explanation pretty well - so it's just the line x = 2 thoughout the diagram?

    I'm now concerned about my answer to the next one, which said:

    rcos(t - pi/6) = 2

    See my diagram and I wait for you to say I've gone wrong on this one as well.
    Attached Images
  3. File Type: pdf img126.pdf (6.2 KB, 80 views)
    • Thread Starter
    Offline

    1
    ReputationRep:
    thnx for the diagram, but should I just draw this?

    If I'm saying that the distance from the polar point is always 2 (and t can take any value between pi and -pi, should I not keep this line continuing (as you would do with x=2 on a cartesian graph)
    • Wiki Support Team
    Offline

    14
    ReputationRep:
    Wiki Support Team
    (Original post by studienka)
    thnx for the diagram, but should I just draw this?

    If I'm saying that the distance from the polar point is always 2 (and t can take any value between pi and -pi, should I not keep this line continuing (as you would do with x=2 on a cartesian graph)
    No, my diagram was just an explanation. It'll be an infinitely long vertical line through (2, 0).
    • Wiki Support Team
    Offline

    14
    ReputationRep:
    Wiki Support Team
    (Original post by studienka)
    I follow the triangle explanation pretty well - so it's just the line x = 2 thoughout the diagram?

    I'm now concerned about my answer to the next one, which said:

    rcos(t - pi/6) = 2

    See my diagram and I wait for you to say I've gone wrong on this one as well.
    Can't work out what you were trying to draw. Try making the substitution s = t - pi/6. Then r cos s = 2 is the same line we had a second ago, relative to the initial line s = 0. But this is the line t - pi/6 = 0, or t = pi/6. So where's the initial line t = 0?
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: August 15, 2007

University open days

  • University of East Anglia
    UEA Mini Open Day Undergraduate
    Fri, 23 Nov '18
  • Norwich University of the Arts
    Undergraduate Open Days Undergraduate
    Fri, 23 Nov '18
  • Edge Hill University
    All Faculties Undergraduate
    Sat, 24 Nov '18
Poll
Black Friday: Yay or Nay?
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

Equations

Best calculators for A level Maths

Tips on which model to get

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.