# Circle Passes through the Origin and Centre Lies on x-axis

We will learn how to find the equation of a circle passes through the origin and centre lies on x-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 passes through the origin and centre lies on x-axis i.e., h = a and k = 0.

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

Circle Passes through the Origin and Centre Lies on x-axis

If a circle passes through the origin and centre lies on x-axis then the abscissa will be equal to the radius of the circle and the y co-ordinate of the centre will be zero. Hence, the equation of the circle will be of the form:

(x - a)$$^{2}$$ + y$$^{2}$$ = a$$^{2}$$

⇒ x$$^{2}$$ + y$$^{2}$$ - 2ax = 0

Solved example on the central form of the equation of a circle passes through the origin and centre lies on x-axis:

1. Find the equation of a circle passes through the origin and centre lies on y-axis at (0, -2).

Solution:

Centre of the lies on y-axis at (0, -2)

Since, circle passes through the origin and centre lies on x-axis then the abscissa will be equal to the radius of the circle and the y co-ordinate of the centre will be zero.

The required equation of the circle passes through the origin and centre lies on y-axis at (0, 2) is

(x + 7)$$^{2}$$ + y$$^{2}$$ = (-7)$$^{2}$$

⇒ x$$^{2}$$ + 14x + 49 + y$$^{2}$$ = 49

⇒ x$$^{2}$$ + y$$^{2}$$ + 14x = 0

2. Find the equation of a circle passes through the origin and centre lies on x-axis at (12, 0).

Solution:

Centre of the lies on x-axis at (12, 0)

Since, circle passes through the origin and centre lies on x-axis then the abscissa will be equal to the radius of the circle and the y co-ordinate of the centre will be zero.

The required equation of the circle passes through the origin and centre lies on x-axis at (12, 0) is

(x - 12)$$^{2}$$ + y$$^{2}$$ = 12$$^{2}$$

⇒ x$$^{2}$$ - 24x + 144 + y$$^{2}$$ = 144

⇒ x$$^{2}$$ + y$$^{2}$$ - 24x = 0

The Circle

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