The equation of a circle with centre C is .
- (i)
Find the coordinates of C and the radius of the circle.
Find the coordinates of C and the radius of the circle.
- (ii)
Show that the point P(9, 13) lies on the circle.
Show that the point P(9, 13) lies on the circle.
- (iii)
A line, l is parallel to the tangent to the circle at P. Q is a point on line l such that P is the midpoint of the line QC.
- (a)
Find the equation of the line l.
- (b)
Explain if the line l will intersect the circle.
- (a)
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Worked solution
9(i)
1. Group the $x$ and $y$ terms and move the constant to the right side
2. Complete the square for $x$ and $y$
3. Simplify to standard circle equation form
4. Extract the centre coordinates and radius
Centre C is .
Radius .
Coordinates of C: , Radius:
9(ii)
1. Substitute the coordinates of P into the left-hand side of the circle's equation.
Substitute and into :
2. Evaluate the expression to show it equals 0.
Since , the point lies on the circle.
Since the substitution yields , the point lies on the circle.
9(iii)(a)
1. Find the coordinates of the centre C from part (i)
From the circle equation , completing the square gives:
Centre is .
2. Find the gradient of the line l
Gradient of
The tangent at is perpendicular to the radius , so its gradient is .
Since line is parallel to the tangent, the gradient of line is also .
3. Find the coordinates of Q
Let be . Since is the midpoint of :
So, is .
4. Find the equation of line l
Using the point and gradient :
9(iii)(b)
1. Determine the perpendicular distance from the centre C to the line $l$
Since line is parallel to the tangent at P, it is perpendicular to the radius CP.
Because P is the midpoint of QC, the points C, P, and Q are collinear.
Thus, the line segment CQ is perpendicular to at Q, meaning the perpendicular distance from the centre C to line is the length CQ.
2. Compare the distance CQ with the radius of the circle to conclude
Since P is the midpoint of QC, .
Since CP is the radius of the circle, is twice the radius ().
Because the perpendicular distance from the centre to line is strictly greater than the radius, the line lies entirely outside the circle and will not intersect it.
The line will not intersect the circle because its perpendicular distance from the centre is greater than the radius.
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