Results 1 to 3 of 3
Like Tree1Thanks
  • 1 Post By frick

Thread: Help working with two points of a circle

  1. #1
    Newbie
    Joined
    Jun 2019
    From
    Kop
    Posts
    1

    Help working with two points of a circle

    Hi all,


    I'm currently preparing to resit a Math exam from a few years back I failed at college in order to graduate. I am weak at Math and haven't done it in sometime. I am having difficulty with the below kind of questions:


    The diameter of a circle starts at (-101, -159) and ends at (-158, 62). What is the circumference of that circle with your answer rounded up to a whole number.


    The answer for the above is 717 1%, according to my Virtual learning environment, but I have no idea why. I tried using the distance formula to get the diameter, then divided the answer by 2 to get the radius which I then put in to the formula for getting the circumference.


    Another question:
    The radius of a circle starts at (173, -102) and ends at (53, -4). What is the area of that circle with your answer rounded up to a whole number.


    The answer for the above question is 75,411 1%. Is this the same as above and then just use the area of a circle formula instead?


    I'd greatly appreciate if someone could run through these questions and explain how to get a (correct) number to plug in to the formulas from the two points.


    Cheers.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Newbie
    Joined
    May 2019
    From
    Kansas
    Posts
    5
    Thanks
    6

    Re: Help working with two points of a circle

    The distance from (-101, -159) to (-152, 62) is $\displaystyle \sqrt{(-101+ 152)^2+ (-159- 62)^2}= \sqrt{51^2+ (-221)^2}= \sqrt{51442}= 226.81$. That is the diameter of the circle and the circumference is $\displaystyle \pi$ times the diameter (there is no reason to divide by 2 to get the radius. Using "$\displaystyle c= 2\pi r$ you would just multiply by 2 again.) so is $\displaystyle 3.1416(225.81)= 712.59$ which, rounded up to a whole number is 713, not "717". I have no idea why they have the "$\displaystyle \pm 1$%". There is nothing said about an error in the measurement. "

    Similarly, the distance from (173, -102) to (53, -4) is $\displaystyle \sqrt{(173- 53)^2+ (-102+ 4)^2}= \sqrt{24004}= 153.932$. The area is $\displaystyle \pi r^2= 3.1415(153.932)^2= 75411$ "rounded up to a whole number". Again, I see no reason for the "$\displaystyle \pm 1$%".
    Thanks from topsquark
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Joined
    Feb 2015
    From
    Ottawa Ontario
    Posts
    2,254
    Thanks
    499

    Re: Help working with two points of a circle

    Quote Originally Posted by frick View Post
    The distance from (-101, -159) to (-152, 62) is $\displaystyle \sqrt{(-101+ 152)^2+ (-159- 62)^2}= \sqrt{51^2+ (-221)^2}= \sqrt{51442}= 226.81$.
    That is the diameter of the circle and the circumference is $\displaystyle \pi$ times the diameter (there is no reason to divide by 2 to get the radius.
    Using "$\displaystyle c= 2\pi r$ you would just multiply by 2 again.) so is $\displaystyle 3.1416(225.81)= 712.59$ which, rounded up to a whole number
    is 713, not "717". I have no idea why they have the "$\displaystyle \pm 1$%". There is nothing said about an error in the measurement. "
    Frick, you made a slight typo: your (-152,62) should be (-158,62).
    That'll lead to ~716.6, thus 717.

    Deano, all you had to do is use George Google and find the Circumference formula....
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 0
    Last Post: Aug 19th 2011, 06:59 AM
  2. Points on a circle.
    Posted in the Algebra Forum
    Replies: 8
    Last Post: Dec 15th 2010, 08:09 AM
  3. Replies: 4
    Last Post: May 2nd 2010, 03:34 PM
  4. Replies: 7
    Last Post: Mar 15th 2010, 04:10 PM
  5. Working out if points belong to a plane or not
    Posted in the Math Topics Forum
    Replies: 2
    Last Post: May 25th 2009, 02:51 AM

/mathhelpforum @mathhelpforum