We will discuss about the general form of the equation of a circle.

Prove that the equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 always represents a circle whose centre is (-g, -f) and radius = \(\sqrt{g^{2} + f^{2} - c}\), where g, f and c are three constants

Conversely, a quadratic equation in x and y of the form x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 always represents the equation of a circle.

We know that the equation of the circle having centre at (h, k) and radius = r units is

(x - h)\(^{2}\) + (y - k)\(^{2}\) = r\(^{2}\)

⇒ x\(^{2}\) + y\(^{2}\) - 2hx - 2hy + h\(^{2}\) + k\(^{2}\) = r\(^{2}\)

⇒ x\(^{2}\) + y\(^{2}\) - 2hx - 2hy + h\(^{2}\) + k\(^{2}\) - r\(^{2}\) = 0

Compare the above equation x\(^{2}\) + y\(^{2}\) - 2hx - 2hy + h\(^{2}\) + k\(^{2}\)
- r\(^{2}\) = 0 with x\(^{2}\) + y\(^{2}\) + 2gx + 2fy +
c = 0 we get, h = -g, k = -f and h\(^{2}\) + k\(^{2}\) - r\(^{2}\) = c

Therefore the equation of any circle can be expressed in the form x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0.

Again, x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0

⇒ (x\(^{2}\) + 2gx + g\(^{2}\)) + (y\(^{2}\) + 2fy + f\(^{2}\)) = g\(^{2}\) + f\(^{2}\) - c

⇒ (x + g)\(^{2}\) + (y +
f)\(^{2}\) = \((\sqrt{g^{2} + f^{2} - c})^{2}\)

⇒ {x - (-g) }\(^{2}\) + {y - (-f) }\(^{2}\) = \((\sqrt{g^{2} + f^{2} - c})^{2}\)

This is of the form (x - h)\(^{2}\) + (y - k)\(^{2}\) = r\(^{2}\) which represents a circle having centre at (- g, -f) and radius \(\sqrt{g^{2} + f^{2} - c}\).

Hence the given equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 represents a circle whose centre is (-g, -f) i.e, (-\(\frac{1}{2}\) coefficient of x, -\(\frac{1}{2}\) coefficient of y) and radius = \(\sqrt{g^{2} + f^{2} - c}\) = \(\sqrt{(\frac{1}{2}\textrm{coefficient of x})^{2} + (\frac{1}{2}\textrm{coefficient of y})^{2} - \textrm{constant term}}\)

**Note:**

(i) The equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 represents a circle of radius = \(\sqrt{g^{2} + f^{2} - c}\).

(ii) If g\(^{2}\) + f\(^{2}\) - c > 0, then the radius of the circle is real and hence the equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 represents a real circle.

(iii) If g\(^{2}\) + f\(^{2}\) - c = 0 then the radius of the circle becomes zero. In this case, the circle reduces to the point (-g, -f). Such a circle is known as a point circle. In other words, the equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 represents a point circle.

(iv) If g\(^{2}\) + f\(^{2}\) - c < 0, the radius of the circle \(\sqrt{g^{2} + f^{2} - c}\) becomes imaginary but the circle is real. Such a circle is called an imaginary circle. In other words, equation x\(^{2}\) + y\(^{2}\) + 2gx + 2fy + c = 0 does not represent any real circle as it is not possible to draw such a circle.

**●** **The Circle**

**Definition of Circle****Equation of a Circle****General Form of the Equation of a Circle****General Equation of Second Degree Represents a Circle****Centre of the Circle Coincides with the Origin****Circle Passes through the Origin****Circle Touches x-axis****Circle Touches y-axis****Circle Touches both x-axis and y-axis****Centre of the Circle on x-axis****Centre of the Circle on y-axis****Circle Passes through the Origin and Centre Lies on x-axis****Circle Passes through the Origin and Centre Lies on y-axis****Equation of a Circle when Line Segment Joining Two Given Points is a Diameter****Equations of Concentric Circles****Circle Passing Through Three Given Points****Circle Through the Intersection of Two Circles****Equation of the Common Chord of Two Circles****Position of a Point with Respect to a Circle****Intercepts on the Axes made by a Circle****Circle Formulae****Problems on Circle**

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