The point A lies on the curve y=xln⁡x2y = x \ln x^2, x>0x > 0. The tangent to the curve at A is parallel to the line y−3x=1y - 3x = 1. The normal to the curve y=xln⁡x2y = x \ln x^2 at A meets the line y−3x=1y - 3x = 1 at B.

  1. (i)

    Find the exact coordinates of A.

  2. (ii)

    Show that the x-coordinate of B can be expressed in the form 110(pe+q)\tfrac{1}{10}\left(p\sqrt{e}+q\right), where pp and qq 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 y=3x+1y = 3x + 1, so its gradient is 33. Since the tangent at A is parallel to this line, the gradient of the tangent at A is 33.

2. Differentiate the equation of the curve

Rewrite the curve equation as y=2xln⁡xy = 2x \ln x (since x>0x > 0).
Using the product rule:
dydx=2ln⁡x+2x(1x)=2ln⁡x+2\frac{dy}{dx} = 2 \ln x + 2x \left(\frac{1}{x}\right) = 2 \ln x + 2

3. Equate the derivative to the gradient and solve for $x$

2ln⁡x+2=32 \ln x + 2 = 3
2ln⁡x=12 \ln x = 1
ln⁡x=12\ln x = \frac{1}{2}
x=e12=ex = e^{\frac{1}{2}} = \sqrt{e}

4. Find the corresponding $y$-coordinate

Substitute x=ex = \sqrt{e} into the curve equation:
y=eln⁡(e)2=eln⁡e=ey = \sqrt{e} \ln(\sqrt{e})^2 = \sqrt{e} \ln e = \sqrt{e}
The exact coordinates of A are (e,e)(\sqrt{e}, \sqrt{e}).

Answer

(e,e)(\sqrt{e}, \sqrt{e})

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 33.
Solving dydx=3\frac{dy}{dx} = 3 gives x=ex = \sqrt{e} and y=ey = \sqrt{e}.
So, AA is (e,e)(\sqrt{e}, \sqrt{e}).
The gradient of the normal at A is mn=−13m_n = -\frac{1}{3}.

2. Form the equation of the normal at A

y−e=−13(x−e)y - \sqrt{e} = -\frac{1}{3}(x - \sqrt{e})

3. Solve simultaneously with the line $y - 3x = 1$ to find the x-coordinate of B

Substitute y=3x+1y = 3x + 1 into the equation of the normal:
3x+1−e=−13(x−e)3x + 1 - \sqrt{e} = -\frac{1}{3}(x - \sqrt{e})
Multiply by 3 to clear the fraction:
9x+3−3e=−x+e9x + 3 - 3\sqrt{e} = -x + \sqrt{e}

4. Rearrange to express $x$ in the required form

10x=4e−310x = 4\sqrt{e} - 3
x=110(4e−3)x = \frac{1}{10}(4\sqrt{e} - 3)
Comparing with 110(pe+q)\frac{1}{10}(p\sqrt{e} + q), we have p=4p = 4 and q=−3q = -3.

Answer

p=4p = 4, q=−3q = -3

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