Past Papers’ Solutions  Assessment & Qualification Alliance (AQA)  AS & A level  Mathematics 6360  Pure Core 1 (6360MPC1)  Year 2012  January  Q#7
Question
A circle with centre C has .
a. Express the equation in the form
b. Write down:
i. the coordinates of C;
ii. the radius of the circle
c. Sketch the circle.
d. A line has the equation , where is a constant.
i. Show that the xcoordinates of any points of intersection of the line and the circle satisfy the equation
.
ii. The equation has equal roots. Show that;
iii. Hence find the values of k for which the line is a tangent to the circle.
Solution
a.
We are given equation of the circle with center C as;
Expression for a circle with center at and radius is;
We can write the given equation of the circle in standard for as follows.
We have the algebraic formula;
For the given case we can rearrange the given equation and compare the given terms with the formula.
For terms containing 
For terms containing 




Therefore, we can deduce that; 



To complete the square we can add and subtract the deduced value of ;
b.
i.
Expression for a circle with center at and radius is;
Comparing the equation from (a) with expression for circle;
i. Coordinates of center of circle .
ii. Radius of circle 5.
c.
d.
i.
If two lines (or a line and a curve) intersect each other at a point then that point lies on both lines i.e. coordinates of that point have same values on both lines (or on the line and the curve). Therefore, we can equate coordinates of both lines i.e. equate equations of both the lines (or the line and the curve).
Equation of the line is;
Equation of the curve is;
Substituting values of in equation of the circle;
ii.
We are given the equation;
We are also given that this equation has equal roots.
For a quadratic equation , the expression for solution is;
Where is called discriminant.
If , the equation will have two distinct roots.
If , the equation will have two identical/repeated roots.
If , the equation will have no roots.
The given equation is a quadratic equation and has equal roots (identical/repeated), therefore;
iii.
If the line is tangent to the circle then solution of equation which satisfies the xcoordinates of any points of intersection of the line and the circle must yield a single solution/equal roots.
We have seen from (ii) that it results in following equation.
To find the possible values of we need to solve .
Therefore, we need to find the critical values for the equation;
Now we have two options.








Therefore, for these values of k equation will yield equal roots and hence line will intersect the circle at a single point and, hence, will be tangent to circle.
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