Past Papers’ Solutions  Assessment & Qualification Alliance (AQA)  AS & A level  Mathematics 6360  Pure Core 1 (6360MPC1)  Year 2017  June  Q#6
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Question
A circle with centre C has .
a. Express this equation in the form
b. Show that the circle touches the yaxis and crosses the xaxis in two distinct points.
c. A line has the equation , where is a constant.
i. Show that the xcoordinates of any points of intersection of the circle and the line satisfy the equation
.
ii. Hence, find the value 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.
We are required to show that there is just a single yintercept and two distinct xintercepts of the circle.
The point at which curve (or line) intercepts xaxis, the value of . So we can find the value of coordinate by substituting in the equation of the curve (or line).
We have found in (a) that equation of the circle can be written as;
Therefore, for coordinates of xintercepts we substitute .
Two distinct values of x indicate that there are two intersection points.
For coordinates of yintercepts, we substitute
Single value of x indicates that there is only one intersection point.
Hence, the given circle intersects xaxis at two distinct points whereas only single intersection point with yaxis.
c.
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.
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 (c:i) that it results in following equation.
To find the possible values of we need to solve above equation.
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.
Since it is a quadratic equation, of it is to yield single/equal roots then its discriminant must be equal to ZERO.
For the above equation to be a quadratic equation;
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