Isaiah's Math Lessons - Circles in Coordinate Geometry


  • Senior Math Wizard Champion (2011, 2012) and Math Count Champion (2011)


    Circle Definition:The locus of a point that moves such that its distance from a fixed point called the center is constant. The constant distance is called the radius, r of the circle.
    General Equation:
    Ax2 + Ay2 + Dx + Ey + F = 0
    Or
    x2 + y2 + Dx + Ey + F = 0

    Standard Equation:
    Circle with center at (h, k) : (x−h)2 + (y−k)2 = r2
    Circle with center at (0, 0) : x2 + y2 = r2

    NOTE: The r variable here stands for the radius of the circle.

    Examples and Detailed Solutions:

    1. What is the general equation of a circle with center at (1, -2) and radius equal to 3?

    Based from the Standard equation of the Circle, we will have the below equation:
    (x−h)2 + (y−k)2 = r2
    But (h, k) is (1,-2) and r = 3
    (x−1)2 + (y−(−2))2 = 32
    (x−1)2 + (y+2)2 = 32
    We know that (a+b)2 = a2 + 2ab + b2
    x2 − 2x + 1 + y2 + 4y + 4 = 9
    Rearranging the Equation, we will have
    x2 + y2 - 2x + 4y – 4 = 0

    2. Find the equation in standard from, center and radius of a circle with equation 4x2+ 4y2−4x+ 12y−15=0

    The trick here is to first make the coefficient of x2 and y2 be equal to 1.
    Therefore, we divide the whole equation by 4. We divide it by 4 since that is the coefficient of both x2 and y2.
    (4x2+ 4y2−4x+ 12y−15) / 4 = 0 = >   x2+ y2− x+3y− 15/4 = 0

    After making the coefficient of x2 and y2 equal to 1, we now group similar terms then completing the square
    ( x2 − x + _ ) + ( y2+ 3y+ _ ) = 15/4 + _ + _
    How do we complete a square? First is we divide the coefficient of the middle term by 2 then square it. For the left side of the equation, the coefficient of the middle term is -1 which is the coefficient of x. We then divide -1 by 2 and then square it which results to 1/4. Same thing goes for the y terms. We divided 3 by 2 then square it which will result to 9/4.

    ( x2− x + 1/4) + ( y2 + 3y + 9/4) = 15/4 + 1/4 + 9/4
    We then factor it to a binomial raised to the 2nd power. We copy the middle term of the expressions above.
    (x−1/2)2 + (y+3/2)2 = ( 5/2)2

    Center (1/2, -3/2) and r = 5/2

    Additional Shortcut: How to find the Center of a Circle in General form having the equation
    x2 + y2 + Dx + Ey + F = 0
    Shortcut Formula (Without Completing the Square)

    Center (−D/2 , -E/2)
    Radius = 1/2√(D2+ E2 − 4F
    )

    Practice Problems:
    1. What is the center of the circle with equation x2+ y2+ 4x−6y+9=0?
    2. For what value of K will make x2+ y2+ 4x−6y+K=0 a point circle? (Clue : a point circle is a circle with radius = 0 )
    3. Find the equation of the circle tangent to the x-axis with center at (1, 2)?
    4. What is the value of the eccentricity of a circle? What does eccentricity means?

    Answers:
    1. C (-2,3)
    2. K = 13
    3. (x−1)2 + (y−2)2 = 22
    4. e=0, it is defined as the parameter which is associated with every conic section. It can be thought as a measure of how much the conic deviates to become a circular figure. Other values of eccentricity are Parabola (e=1), Ellipse (0 < e < 1) and Hyperbola (e > 1).


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