The Circle
ICSE · Class 11 · Mathematics
All formulas from The Circle in ICSE Class 11 Mathematics. Key equations, constants, and identities for board exam preparation.
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1. Core Definitions and Terms
Circle = locus of all points in a plane equidistant from a fixed point
Standard Equation of a Circle: (x-h)^2+(y-k)^2=r^2
Equation of circle centred at origin: x^2+y^2=r^2
General Equation of a Circle: x^2+y^2+2gx+2fy+c=0
Diameter Equation of a Circle: (x-x1)(x-x2)+(y-y1)(y-y2)=0
2. Standard Form and Quick Conversions
If centre is (0,2) and radius is 2, then x^2+(y-2)^2=4
If centre is (-2,3) and radius is 4, then (x+2)^2+(y-3)^2=16
If centre is (0,0) and radius is a, then x^2+y^2=a^2
If centre is (-a,-b) and radius is sqrt(a^2-b^2), then x^2+y^2+2ax+2by+2b^2=0
3. General Equation of a Circle
x^2+y^2+2gx+2fy+c=0
Centre of x^2+y^2-6x+4y-12=0 is (3,-2)
Radius of x^2+y^2-6x+4y-12=0 is 5
For 2x^2+2y^2+3x+4y+9/8=0, divide by 2 first
4. Diameter Equation
(x-x1)(x-x2)+(y-y1)(y-y2)=0
If diameter endpoints are (1,3) and (4,5), equation is (x-1)(x-4)+(y-3)(y-5)=0
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