Q&A for How to Find the Center of a Circle

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  • Question
    How do you find the center of a circle if you're only given the equation?
    David Jia
    David Jia
    Math Tutor
    David Jia is an Academic Tutor and the Founder of LA Math Tutoring, a private tutoring company based in Los Angeles, California. With over 10 years of teaching experience, David works with students of all ages and grades in various subjects, as well as college admissions counseling and test preparation for the SAT, ACT, ISEE, and more. After attaining a perfect 800 math score and a 690 English score on the SAT, David was awarded the Dickinson Scholarship from the University of Miami, where he graduated with a Bachelor’s degree in Business Administration. Additionally, David has worked as an instructor for online videos for textbook companies such as Larson Texts, Big Ideas Learning, and Big Ideas Math.
    David Jia
    Math Tutor
    Expert Answer
    The equation of a circle is (x-h)²+(y-k)²=r². The coordinates for the center of the circle are (h, k), so to find the center of the circle, solve for h and k.
  • Question
    How do you find the center of the circle if you're only given the endpoints of the diameter?
    David Jia
    David Jia
    Math Tutor
    David Jia is an Academic Tutor and the Founder of LA Math Tutoring, a private tutoring company based in Los Angeles, California. With over 10 years of teaching experience, David works with students of all ages and grades in various subjects, as well as college admissions counseling and test preparation for the SAT, ACT, ISEE, and more. After attaining a perfect 800 math score and a 690 English score on the SAT, David was awarded the Dickinson Scholarship from the University of Miami, where he graduated with a Bachelor’s degree in Business Administration. Additionally, David has worked as an instructor for online videos for textbook companies such as Larson Texts, Big Ideas Learning, and Big Ideas Math.
    David Jia
    Math Tutor
    Expert Answer
    If you're given two points that are the endpoints of the diameter of the circle, the midpoint of that line will be the center of the circle.
  • Question
    In the first method, what do I do if the chords are of different lengths?
    Community Answer
    Community Answer
    It's not easy to construct parallel chords of equal length. In practice, it would be a process of trial and error until you get the chords you need. But the real goal here is to find the center of a circle, and here's a way to do it without worrying about equal and parallel chords: (1) draw any two or more chords; (2) perpendicularly bisect each chord (using either a compass or a ruler and right triangle; (3) the perpendicular bisectors will intersect at the circle's center.
  • Question
    What is the center of a circle?
    Donagan
    Donagan
    Top Answerer
    It is the one point that is equidistant from all points on the circumference.
  • Question
    If I use a compass to draw a circle, is the pivot point the center?
    Donagan
    Donagan
    Top Answerer
    Yes, the point of a compass is placed at the center of a circle drawn by the compass. The object of this article, however, is to find the center of a circle that you have not drawn.
  • Question
    How do I find the center of a circle using a pencil only, nothing else?
    Donagan
    Donagan
    Top Answerer
    It cannot be done accurately. Imagine trying to do it with a very large circle.
  • Question
    Why not find the longest distance in the circle with a straight edge, divide the measurement in half, put a dot at that measurement you're done? No marks on the page, so simple.
    Donagan
    Donagan
    Top Answerer
    While your method is quick and easy, it's not guaranteed to be absolutely accurate. There's a bit of guesswork involved. The methods described above introduce an element of guesswork, too, but they're designed to come extremely close to the center of the circle.
  • Question
    How do I find the center of a circular piece of wood? If I use your first method, how do I determine if lines AB and CD are parallel?
    Community Answer
    Community Answer
    Measure the distance between the chords at opposite sides of the circle. If the distances are equal, the chords are parallel. Also, you can measure the lengths of the two diagonal lines (AD and BC). If those lengths are equal, the chords are parallel. In either case the chords do not have to be of the same length.
  • Question
    I want to make a coin ring and need to find the center of a quarter dollar. It is tough to draw a lot of lines on the uneven surface. Any ideas?
    Donagan
    Donagan
    Top Answerer
    Trace the outline of the coin on a piece of (heavy) paper. Locate the center of the paper circle. Punch a small hole through the center. Cut out the circle. Place it directly over the coin. Use a fine-tipped marking pen to mark the center through the paper.
  • Question
    Is the point of intersection of two diameters of the same circle its centre?
    Community Answer
    Community Answer
    Yes, the point of intersection is its center.
  • Question
    Can't you draw two non parallel cords, find the center of each cord, draw perpendicular lines, they should intersect at the center?
    Donagan
    Donagan
    Top Answerer
    Yes, that is true. The perpendicular bisector of any chord of a circle passes through the center of the circle.
  • Question
    What is the run in inches for a rise of 5 inches at a 3% grade?
    Donagan
    Donagan
    Top Answerer
    3% = 3/100 = 5" / x, where x is the run (in inches). By cross-multiplying, 3x = 500", and x = 166.67 inches.
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