We will learn how to find the equation of a circle touches y-axis.

The equation of a circle with centre at (h, k) and radius equal to a, is (x - h)\(^{2}\) + (y - k)\(^{2}\) = a\(^{2}\).

When the circle touches y-axis i.e., h = a.

Then the equation (x - h)\(^{2}\) + (y - k)\(^{2}\) = a\(^{2}\) becomes (x - a)\(^{2}\) + (y - k)\(^{2}\) = a\(^{2}\)

If a circle touches the y-axis, then the x-co-ordinate of the centre will be equal to the radius of the circle.

Hence, the equation of
the circle will be of the form (x - a)\(^{2}\) + (y - k)\(^{2}\) =
a\(^{2}\)

Let C (h, k) be the centre of the circle. Since the circle touches the y-axis, therefore, a = h

Hence the equation of the circle is (x - a)\(^{2}\) + (y - k)\(^{2}\) = a\(^{2}\) ⇒ x\(^{2}\) + y\(^{2}\) – 2ax – 2ky + k\(^{2}\) = 0

Solved examples on the central form of the equation of a circle touches y-axis:

**1.** Find the equation of a circle whose y-coordinate of the
centre is -7 and radius is 3 units also touches the y-axis.

**Solution:**

The required equation of the circle whose y-coordinate of the centre is -7 and radius is 3 units also touches the y-axis is (x - 3)\(^{2}\) + (y + 7)\(^{2}\) = 3\(^{2}\), [Since radius is equal to the x-coordinate of the centre]

⇒ x\(^{2}\) – 6x + 9 + y\(^{2}\) + 14y + 49 = 9

⇒ x\(^{2}\) + y\(^{2}\) - 6x + 14y + 49 = 0

**2.** Find the equation of a circle whose radius is 9 units and y-coordinate
of the centre is -6 and also touches the y-axis.

**Solution:**

The required equation of the circle whose radius is 9 units and y-coordinate of the centre is -6 and also touches the x-axis is (x - 9)\(^{2}\) + (y + 6)\(^{2}\) = 9\(^{2}\), [Since radius is equal to the x-coordinate of the centre]

⇒ x\(^{2}\) - 18x + 81 + y\(^{2}\) + 12y + 36 = 81

⇒ x\(^{2}\) + y\(^{2}\) - 18x + 12y + 36 = 0

**●** **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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