It is given that y=xxyy = x^{xy}, where x>0x > 0, y>0y > 0.

  1. (a)

    Show that dydx=y2(1+ln⁡x)1−yln⁡x\frac{\text{d}y}{\text{d}x} = \frac{y^2 (1 + \ln x)}{1 - y \ln x}.

  2. (b)

    Hence find the coordinates of the point on the curve y=xxyy = x^{xy} whose tangent is parallel to the yy-axis.

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Worked solution

3(b)

1. Translate 'tangent parallel to the y-axis' into a condition

A tangent parallel to the yy-axis is vertical, so dydx\dfrac{dy}{dx} is undefined. Hence the denominator of dydx=y2(1+ln⁡x)1−xyln⁡x\dfrac{dy}{dx}=\dfrac{y^2(1+\ln x)}{1-xy\ln x} must be zero:
1−xyln⁡x=0⇒xyln⁡x=1.1 - xy\ln x = 0 \quad\Rightarrow\quad xy\ln x = 1.

2. Use the original curve relation to find y

Taking logarithms of y=xxyy = x^{xy} gives ln⁡y=xyln⁡x\ln y = xy\ln x. Substituting xyln⁡x=1xy\ln x = 1:
ln⁡y=1⇒y=e.\ln y = 1 \quad\Rightarrow\quad y = e.

3. Find the x-coordinate using a graphing calculator

Substitute y=ey = e into xyln⁡x=1xy\ln x = 1:
e xln⁡x=1⇒xln⁡x=1e.e\,x\ln x = 1 \quad\Rightarrow\quad x\ln x = \tfrac{1}{e}.
Solving with a GC gives x≈1.32x \approx 1.32.
Hence the point is (1.32, e)≈(1.32, 2.72)(1.32,\ e) \approx (1.32,\ 2.72).

Answer

The point is (1.32, e)(1.32,\ e), i.e. approximately (1.32, 2.72)(1.32,\ 2.72) (to 3 s.f.).

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