Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie
Hi there,
There is an issue with role permissions that is being worked on at the moment.
If you are having trouble with access or permissions on regional forums please post here to get access: https://www.boards.ie/discussion/2058365403/you-do-not-have-permission-for-that#latest

Important Tech Graph Question!

  • 16-06-2011 4:19pm
    #1
    Eoin_Sheehy
    Closed Accounts Posts: 274 ✭✭


    I've totally forgotten how to draw a tangent to an ellipse from an external point, could anyone enlighten me?


Comments

  • #2
    P H A 7V T O M
    Closed Accounts Posts: 1,006 ✭✭✭P H A 7V T O M


    Close your eyes, breathe in slowly. Become the pencil. You, EoinSheely, are that 6 inches of carbon inside that Faber Castel pencil. Theres your tangent!

    Hrmm with a few word changes (note: pencil and carbon) that sentance can become a whole new set of instructions!

    Heres an ACTUAL guide

    http://www.mathopenref.com/consttangent.html


  • #3
    Eoin_Sheehy
    Closed Accounts Posts: 274 ✭✭Eoin_Sheehy


    Thank you very much, I'm now being a pencil and drawing a massive penis on one of y school books.


  • #4
    P H A 7V T O M
    Closed Accounts Posts: 1,006 ✭✭✭P H A 7V T O M


    Eoin_Sheehy wrote: »
    Thank you very much, I'm now being a pencil and drawing a massive penis on one of y school books.

    Hey! I also gave you a real helpful link!


  • #5
    FatRat
    Registered Users, Registered Users 2 Posts: 3,194 ✭✭✭FatRat


    Put your compas on the external point. Draw an arc from the closest focal point to the external point.
    Get the length of the major axis and, from the other focal point draw an arc which intersects the arc you already have.

    There is another step to it but I forget what it is. :cool: But this is pretty much all of it!


  • #6
    Eoin_Sheehy
    Closed Accounts Posts: 274 ✭✭Eoin_Sheehy


    P H A 7V T O M wrote: »
    Hey! I also gave you a real helpful link!

    I also realized the yours won't work as it's for a circle, need it for an ellipse.


  • Advertisement
  • #7
    MathsManiac
    Registered Users, Registered Users 2 Posts: 1,595 ✭✭✭MathsManiac


    There will be two tangents, of course. I'm sure there's more than one way to do it, but this'll work:

    Suppose your external point is P, and the nearest focus of the ellipse is F, and the farther one is F'.
    1. Draw a circle c centred at P and passing through F.
    2. Draw an arc with centre F' and radius equal to the major axis of the ellipse, cutting the circle c at the points A and B.
    3. Join F' ot A and F' to B. These two lines cross the ellipse at C and D, which are required points of contact for the tangents from P.


  • #8
    fullback4glin
    Closed Accounts Posts: 383 ✭✭fullback4glin


    P H A 7V T O M wrote: »
    Close your eyes, breathe in slowly. Become the pencil. You, EoinSheely, are that 6 inches of carbon inside that Faber Castel pencil. Theres your tangent!

    Hrmm with a few word changes (note: pencil and carbon) that sentance can become a whole new set of instructions!

    Heres an ACTUAL guide

    http://www.mathopenref.com/consttangent.html

    That's a tangent to a circle, not an ellipse.


Advertisement