# ✅ Circle Formulas ⭐️⭐️⭐️⭐️⭐

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1. General equation of a Circle

x2 + y2 + 2gx + 2fy + c = 0
(i) Centre of a general equation of a circle is (-g, -f)

(ii) If the centre is origin then the equation of a circle is x2 + y2 = r2

3. Diametral Form

If (x1, y1) and (x2, y2) be the extremities of a diameter, then the equation of circle is (x – x1) (x – x2) + (y – y1) (y – y2) = 0

4. The parametric equations of a Circle

• The parametric equations of a circle x2 + y2 = r2 are x = r cos θ, y = r sin θ.
• The parametric equations of the circle (x – h)2 + (y – k)2 = r2 are x = h + r cos θ, y = k + r sin θ.
• Parametric equations of the circle x2 + y2 + 2gx + 2fy + c = 0 are

5. Position of a Point with respect to a Circle

The following formulae are also true for Parabola and Ellipse.
S1 > 0 ⇒ Point is outside the circle.
S1 = 0 ⇒ Point is on the circle.
S1 < 0 ⇒ Point is inside the circle.

6. Length of the intercept made by the circle on the line

8. Condition of tangency

10. Equation of Tangent T = 0

(i) The equation of tangent to the circle x2 + y2 + 2gx + 2fy + c = 0 at a point (x1 y1) is xx1 + yy1 + g(x + x1) + f(y + y1) + c = 0

(ii) The equation of tangent to circle x2 + y2 = a2 at point (x1, y1) is xx1 + yy1 = a2

(iii) Slope Form:
From condition of tangency for every value of m, the line

11. Equation of Normal

• The equation of normal to the circle x2 + y2 + 2gx + 2fy + c = 0 at any point (x1, y1) is

The equation of normal to the circle x2 + y2 = a2 at any point (x1, y1) is xy1 – x1y = 0

15. Director Circle

The locus of the point of intersection of two perpendicular tangents to a circle is called the Director circle.
Let the circle be x2 + y2 = a2, the equation of director circle is x2 + y2 = 2a2, director circle is a concentric circle whose radius is

16. Equation of Polar and coordinates of Pole

• Equation of polar is T = 0
• Pole of polar Ax + By + C = 0 with respect to circle x2 + y2 = a2 is

17. Equation of a chord whose middle point is given T = S1

18. The equation of the circle passing through the points of intersection of the circle S = 0 and line L = 0 is S + λL = 0.

19. Diameter of a circle

The diameter of a circle x2 + y2 = r2 corresponding to the system of parallel chords y = mx + c is x + my = 0.

20. Equation of common chord S1 – S2 = 0

21. Two circles with radii r1, r2 and d be the distance between their centres then the angle of intersection θ between them is given by cos θ =

22. Condition of Orthogonality

2g1g2 + 2f1f2 = c1 + c2

23. Relative position of two circles and No. of common tangents

Let C1 (h1, k1) and C2 (h2, k2) be the centre of two circle and r1, r2 be their radius then

• C1C2 > r1 + r2 ⇒ do not intersect or one outside the other ⇒ 4 common tangents
• C1C2 < |r1 – r2| ⇒ one inside the other => 0 common tangent
• C1C2 = r1 + r2 ⇒ external touch ⇒ 3 common tangents
• C1C2 = |r1 – r2| ⇒ internal touch ⇒ 1 common tangent
• |r1 – r2| < C1C2 < r1 + r2 ⇒ intersection at two real points ⇒ 2 common tangents

24. Equation of the common tangents at point of contact S1 – S2 = 0.

25. Pair of point of contact

The point of contact divides C1C2 in the ratio r1: r2 internally or externally as the case may be.

• Definition of Radical axis: Locus of a point from which length of tangents to the circles are equal, is called radical axis.
• radical axis is S – S’ = 0
• If S1 = 0, S2 = 0 and S3 = 0 be any three given circles then the radical centre can be obtained by solving any two of the following equations
S1 – S2 = 0, S2 – S3 = 0, S3 – S1 = 0.

27. S1 – S2 = 0 represent equation of all i.e. Radical axis, common axis, common tangent i.e.

when circle are not in touch → Radical axis
when circle are in touch → Common tangent
when circle are intersecting → Common chord

28. Let θ1 and θ2 are two points lies on circle x2 + y2 = a2, then equation of line joining these two points is

29. Limiting Point of co-axial system of circles:

Limiting point of a system of co-axial circles are the centres of the point circles belonging to the family. Two such point of a co-axial are