Past Papers’ Solutions  Edexcel  AS & A level  Mathematics  Core Mathematics 1 (C16663/01R)  Year 2014  June  Q#11
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Question
A sketch of part of the curve C with equation
, x>0
is shown in Figure.
Point A lies on C and has an x coordinate equal to 2
a. Show that the equation of the normal to C at A is y = –2x + 7.
The normal to C at A meets C again at the point B, as shown in Figure.
b. Use algebra to find the coordinates of B.
Solution
a.
We are required to find equation of normal to curve C at point A(2,y).
To find the equation of the line either we need coordinates of the two points on the line (TwoPoint form of Equation of Line) or coordinates of one point on the line and slope of the line (PointSlope form of Equation of Line).
We already have xcoordinates of a point on the normal A(2,y). Therefore, we need ycoordinates of a point on the normal A(2,y) and slope of the normal to write its equation.
First we find ycoordinate of the point A on the normal. Since this point also lies on the curve, we can utilize equation of the curve to find ycoordinate of point A(2,y).
If a point lies on the curve (or the line), the coordinates of that point satisfy the equation of the curve (or the line).
We are given that point A(2,y) lies on the curve which has equation;
We substitute coordinates of point A in the equation of the curve C;
Hence, coordinates of point A(2,3).
Next we find slope of the normal to the curve C.
If a line is normal to the curve , then product of their slopes and at that point (where line is normal to the curve) is;
Therefore;
We can find slope of normal to the curve at point A(2,y) if we have slope of curve at the same point.
We need to find the gradient of the curve C at point A(2,y).
Gradient (slope) of the curve at the particular point is the derivative of equation of the curve at that particular point.
Gradient (slope) of the curve
Therefore, we need
We are given that;
We are required to find
Gradient (slope) of the curve is the derivative of equation of the curve. Hence gradient of curve
Therefore;
Rule for differentiation is of
Rule for differentiation is of
Rule for differentiation is of
We need to find the gradient of the curve when x=2.
Gradient (slope) of the curve at the particular point is the derivative of equation of the curve at that particular point.
Gradient (slope)
We have already found that;
Substitute
Hence, gradient of the curve C at points (2,y) is;
Now we can find slope of normal to the curve at point (2,y).
Now we can write equation of the normal.
PointSlope form of the equation of the line is;
b.
We are given that the normal to C at A meets C again at the point B, as shown in Figure.
Therefore, normal and the curve intersect each other at point B and we are required to find the coordinates of B.
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
Equation of the normal is;
Equation of the curve is;
Equating both equations;
Now we have two options.









Two values of x indicate that there are two intersection points.
We know that
With xcoordinate of point of intersection of two lines (or line and the curve) at hand, we can find the ycoordinate of the point of intersection of two lines (or line and the curve) by substituting value of xcoordinate of the point of intersection in any of the two equations.
We choose equation of the curve;
Hence, coordinates of point B are
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