It is given that x=−2x = -2 and y=−1y = -1 are tangents to a circle.
The xx-coordinate and yy-coordinate of the centre of the circle are positive.
The line 3y=2x+53y = 2x + 5 is a normal to the circle.

  1. (a)

    Show that the centre of the circle is (2,3)(2, 3).

  2. (b)

    Find the equation of the circle.

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Worked solution

6(a)

1. Express the radius of the circle in terms of the coordinates of its centre.

Let the centre of the circle be (h,k)(h, k) and its radius be rr.
Since x=−2x = -2 is a tangent and h>0h > 0, the distance from the centre to this tangent is r=h−(−2)=h+2r = h - (-2) = h + 2.
Since y=−1y = -1 is a tangent and k>0k > 0, the distance from the centre to this tangent is r=k−(−1)=k+1r = k - (-1) = k + 1.

2. Form an equation relating the coordinates of the centre.

Equating the two expressions for the radius:
h+2=k+1h + 2 = k + 1
k=h+1k = h + 1

3. Use the property of the normal to form a second equation.

A normal to a circle always passes through its centre. Therefore, the centre (h,k)(h, k) lies on the line 3y=2x+53y = 2x + 5.
Substituting x=hx = h and y=ky = k into the equation:
3k=2h+53k = 2h + 5

4. Solve the simultaneous equations to find the centre.

Substitute k=h+1k = h + 1 into 3k=2h+53k = 2h + 5:
3(h+1)=2h+53(h + 1) = 2h + 5
3h+3=2h+53h + 3 = 2h + 5
h=2h = 2
Substitute h=2h = 2 back into k=h+1k = h + 1:
k=2+1=3k = 2 + 1 = 3
Since h=2>0h = 2 > 0 and k=3>0k = 3 > 0, this satisfies the given conditions.
Thus, the centre of the circle is (2,3)(2, 3) (shown).

Answer

The centre of the circle is (2,3)(2, 3).

6(b)

1. Find the radius of the circle

The radius is the perpendicular distance from the centre (2,3)(2, 3) to the tangent x=−2x = -2 (or y=−1y = -1).
r=2−(−2)=4r = 2 - (-2) = 4

2. Form the equation of the circle

Using the standard form (x−a)2+(y−b)2=r2(x - a)^2 + (y - b)^2 = r^2:
(x−2)2+(y−3)2=42(x - 2)^2 + (y - 3)^2 = 4^2
(x−2)2+(y−3)2=16(x - 2)^2 + (y - 3)^2 = 16

Answer

(x−2)2+(y−3)2=16(x - 2)^2 + (y - 3)^2 = 16

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