Given that we need to find the equation of the circle whose centre is (1, 2) and passing through the point (4, 6).

We know that the radius of the circle is the distance between the centre and any point on the radius. So, we find the radius of the circle.

We know that the distance between the two points (x₁,y₁) and (x₂,y₂) is .

Let us assume r is the radius of the circle.

⇒

⇒

⇒

⇒

⇒ r = 5 units ..... (1)

We know that the equation of the circle with centre (p, q) and having radius ‘r’ is given by:

⇒ (x - p)² + (y - q)² = r²

Now we substitute the corresponding values in the equation:

⇒ (x - 1)² + (y - 2)² = 5²

⇒ x² - 2x + 1 + y² - 4y + 4 = 25

⇒ x² + y² - 2x - 4y - 20 = 0.

∴The equation of the circle is x² + y² - 2x - 4y - 20 = 0.