It is given that and are tangents to a circle.
The -coordinate and -coordinate of the centre of the circle are positive.
The line is a normal to the circle.
- (a)
Show that the centre of the circle is .
- (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 and its radius be .
Since is a tangent and , the distance from the centre to this tangent is .
Since is a tangent and , the distance from the centre to this tangent is .
2. Form an equation relating the coordinates of the centre.
Equating the two expressions for the radius:
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 lies on the line .
Substituting and into the equation:
4. Solve the simultaneous equations to find the centre.
Substitute into :
Substitute back into :
Since and , this satisfies the given conditions.
Thus, the centre of the circle is (shown).
The centre of the circle is .
6(b)
1. Find the radius of the circle
The radius is the perpendicular distance from the centre to the tangent (or ).
2. Form the equation of the circle
Using the standard form :
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