1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Checking: Circle Equations Questions

  1. Feb 3, 2008 #1
    [SOLVED] Checking: Circle Equations Questions

    Last time I got people to check my homework here, I got things marked wrong when they where said to be right. Lets see if it happens again...

    1. The problem statement, all variables and given/known data

    (1) The circle C has centre (3, 4) and passes through the point (8, 8).
    Find an equation for C.

    (2) Find the centre and radius of the circle x² + y² + 4x + 6y – 17 = 0

    (3) A(4, 0) and B(3, 5) are the end points of a diameter of the circle C.
    Find (a) the exact length of AB,
    (b) the coordinates of the midpoint P of AB,
    (c) an equation for the circle C.

    2. Relevant equations

    None

    3. The attempt at a solution

    Right here is my working for (1)

    Since we know the circle centre and a point the circle passes through, I can use pythag to work on the radius right? It seems a little too simple to work, but I can't see anything wrong with it. After drawing a clear diagram;

    (4+8)² + 5² = r²
    r² = 144 + 25
    r² = 169
    r = 13

    Therefore the equation must be (x-3)² + (y-4)² = 13
    Correct?

    Now, my working for (2);

    The equation needs to be factorised, so I have to "complete the square" to get the stuff inside the brackets. This is usually where I go wrong, so check this bit carefully please!

    (x+2)² - 4 + (y+3)² - 9 - 17 = 0

    Rearraged:

    (x+2)² + (y+3)² = 17

    Ah, I have to go now, but I'll post the answers to 3 quickly:

    a) I got 5.09
    b) x = 3.5
    y = 2.2
    and
    c) (x+3.5)² + (y+2.2)² = 5.09

    Sorry about that lack of working out. Thank you for your time if you've read this far!
     
  2. jcsd
  3. Feb 3, 2008 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    1) looks fine. I would make 2) to be (x+2)^2+(y+3)^2=30. Is that just a typo? For 3) they ask for the EXACT value of the length of AB, so I would write sqrt(26) rather than the approximation 5.09.
     
  4. Feb 4, 2008 #3
    Thanks! Gotta hand it in later today, so we'll see if I've got them right.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Checking: Circle Equations Questions
  1. Circle Equation (Replies: 4)

  2. Circle Question (Replies: 3)

  3. Equations of Circles (Replies: 1)

Loading...