The point A lies on the curve , . The tangent to the curve at A is parallel to the line . The normal to the curve at A meets the line at B.
- (i)
Find the exact coordinates of A.
- (ii)
Show that the x-coordinate of B can be expressed in the form , where and are integers to be found.
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Worked solution
9(i)
1. Find the gradient of the line $y - 3x = 1$
The line is , so its gradient is . Since the tangent at A is parallel to this line, the gradient of the tangent at A is .
2. Differentiate the equation of the curve
Rewrite the curve equation as (since ).
Using the product rule:
3. Equate the derivative to the gradient and solve for $x$
4. Find the corresponding $y$-coordinate
Substitute into the curve equation:
The exact coordinates of A are .
9(ii)
1. Find the coordinates of A and the gradient of the normal at A (using results from part i)
From part (i), the gradient of the tangent at A is .
Solving gives and .
So, is .
The gradient of the normal at A is .
2. Form the equation of the normal at A
3. Solve simultaneously with the line $y - 3x = 1$ to find the x-coordinate of B
Substitute into the equation of the normal:
Multiply by 3 to clear the fraction:
4. Rearrange to express $x$ in the required form
Comparing with , we have and .
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