It is given that f(x)=x+m2x−2\mathrm{f}(x) = x + \frac{m^2}{x-2}, where 0<m<10 < m < 1.

  1. (i)

    Sketch the graph of y=f(x)y = \mathrm{f}(x), showing clearly the coordinates of the turning points and the equation(s) of any asymptote(s).

  2. (ii)

    By inserting a suitable graph to your sketch in (i), find the set of values of kk, in terms of mm, for which the equation x2−(2+k)x+(m2+2k)=0x^2 - (2+k)x + (m^2 + 2k) = 0 has two distinct positive roots.

  3. (iii)

    The curve y=f(x)y = \mathrm{f}(x) undergoes the transformations AA, BB and CC in succession:

    AA: A translation of −2-2 units in the direction of xx-axis,

    BB: A stretch parallel to the xx-axis with scale factor of 12\frac{1}{2}, and

    CC: A translation of −2-2 units in the direction of yy-axis.

    Given that the resulting curve is y=2x+18xy = 2x + \frac{1}{8x}, find the value of mm.

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Worked solution

9(i)

1. Identify the vertical and oblique asymptotes of the graph.

For y=x+m2x−2y = x + \frac{m^2}{x-2}, the fraction is undefined when the denominator is zero.
As x→2x \to 2, y→±∞y \to \pm\infty, so the vertical asymptote is x=2x = 2.
As x→±∞x \to \pm\infty, m2x−2→0\frac{m^2}{x-2} \to 0, so the curve approaches the line y=xy = x.
The oblique asymptote is y=xy = x.

2. Find the $x$-coordinates of the turning points by differentiating.

To find the turning points, set dydx=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0:
dydx=1−m2(x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = 1 - \frac{m^2}{(x-2)^2}
1−m2(x−2)2=0  ⟹  (x−2)2=m21 - \frac{m^2}{(x-2)^2} = 0 \implies (x-2)^2 = m^2
Since m>0m > 0, taking the square root gives:
x−2=±m  ⟹  x=2−m or x=2+mx - 2 = \pm m \implies x = 2 - m \text{ or } x = 2 + m

3. Calculate the corresponding $y$-coordinates of the turning points.

Substitute xx back into the original equation to find yy:
When x=2−mx = 2 - m:
y=(2−m)+m2(2−m)−2=2−m−m=2−2my = (2 - m) + \frac{m^2}{(2 - m) - 2} = 2 - m - m = 2 - 2m
When x=2+mx = 2 + m:
y=(2+m)+m2(2+m)−2=2+m+m=2+2my = (2 + m) + \frac{m^2}{(2 + m) - 2} = 2 + m + m = 2 + 2m
The turning points are (2−m,2−2m)(2-m, 2-2m) and (2+m,2+2m)(2+m, 2+2m).

4. Deduce the sketch properties using the given condition $0 < m < 1$.

Since 0<m<10 < m < 1, we have 2−m>1>02-m > 1 > 0 and 2−2m>02-2m > 0.
This means the local maximum (2−m,2−2m)(2-m, 2-2m) lies in the first quadrant, to the left of the asymptote x=2x = 2.
The local minimum (2+m,2+2m)(2+m, 2+2m) lies to the right of x=2x = 2.
The yy-intercept is at y=−m22y = -\frac{m^2}{2} (which is negative).
With these features, sketch the two branches bounded by x=2x=2 and y=xy=x.

Answer

Asymptotes: x=2x = 2 and y=xy = x
Turning points: (2−m,2−2m)(2-m, 2-2m) [local maximum] and (2+m,2+2m)(2+m, 2+2m) [local minimum]
(The sketch should show a rational curve with two branches separated by the vertical asymptote x=2x = 2, bounded by the oblique asymptote y=xy = x. Since 0<m<10 < m < 1, the local maximum (2−m,2−2m)(2-m, 2-2m) is strictly in the first quadrant.)

9(ii)

1. Manipulate the given equation to relate it to the function $\mathrm{f}(x)$.

x2−(2+k)x+(m2+2k)=0x^2 - (2+k)x + (m^2 + 2k) = 0
x2−2x+m2−kx+2k=0x^2 - 2x + m^2 - kx + 2k = 0
x2−2x+m2=k(x−2)x^2 - 2x + m^2 = k(x-2)
Dividing by (x−2)(x-2) since x=2x=2 is an asymptote and not a root:
x2−2x+m2x−2=k\frac{x^2 - 2x + m^2}{x-2} = k
x+m2x−2=kx + \frac{m^2}{x-2} = k
f(x)=k\mathrm{f}(x) = k
The roots of the equation correspond to the xx-coordinates of the intersection points between the curve y=f(x)y = \mathrm{f}(x) and the horizontal line y=ky = k.

2. State the key features of the graph of $y = \mathrm{f}(x)$ derived from part (i).

From part (i), setting f′(x)=1−m2(x−2)2=0\mathrm{f}'(x) = 1 - \frac{m^2}{(x-2)^2} = 0 gives x=2−mx = 2-m and x=2+mx = 2+m.
The local maximum point is (2−m,2−2m)(2-m, 2-2m).
The local minimum point is (2+m,2+2m)(2+m, 2+2m).
The yy-intercept is found by setting x=0x=0:
f(0)=0+m2−2=−m22\mathrm{f}(0) = 0 + \frac{m^2}{-2} = -\frac{m^2}{2}

3. Determine the values of $k$ that give two positive roots on the left branch of the curve.

For the equation to have two distinct positive roots, the line y=ky=k must intersect the curve at two distinct points where x>0x>0.
On the left branch (x<2x < 2), the curve crosses the yy-axis at y=−m22y = -\frac{m^2}{2}, rises to the maximum at y=2−2my = 2-2m, and then falls towards the vertical asymptote x=2x=2.
To intersect this branch at two points with x>0x > 0, the horizontal line y=ky = k must lie strictly above the yy-intercept and below the maximum.
Thus, we require:
−m22<k<2−2m-\frac{m^2}{2} < k < 2-2m

4. Determine the values of $k$ that give two positive roots on the right branch of the curve and combine the results.

The right branch of the curve (x>2x > 2) lies entirely in the region where xx is positive. It has a local minimum at y=2+2my = 2+2m.
Any horizontal line y=ky = k strictly above this minimum will intersect the right branch at two distinct points, both of which will have x>2>0x > 2 > 0.
Thus, we also have valid solutions when:
k>2+2mk > 2+2m
Combining these, the complete set of values is −m22<k<2−2m or k>2+2m-\frac{m^2}{2} < k < 2-2m \text{ or } k > 2+2m.

Answer

−m22<k<2−2m or k>2+2m-\frac{m^2}{2} < k < 2-2m \text{ or } k > 2+2m

9(iii)

1. Apply transformation $A$ to the original curve equation.

Transformation AA is a translation of −2-2 units in the xx-direction, which means replacing xx with x+2x + 2.
y=f(x+2)=(x+2)+m2(x+2)−2=x+2+m2xy = \mathrm{f}(x+2) = (x+2) + \frac{m^2}{(x+2)-2} = x + 2 + \frac{m^2}{x}

2. Apply transformations $B$ and $C$ to find the final equation.

Transformation BB is a stretch parallel to the xx-axis with scale factor 12\frac{1}{2}, which requires replacing xx with 2x2x.
y=(2x)+2+m22x=2x+2+m22xy = (2x) + 2 + \frac{m^2}{2x} = 2x + 2 + \frac{m^2}{2x}

Transformation CC is a translation of −2-2 units in the yy-direction, which means subtracting 22 from the entire function.
y=(2x+2+m22x)−2=2x+m22xy = \left(2x + 2 + \frac{m^2}{2x}\right) - 2 = 2x + \frac{m^2}{2x}

3. Compare the transformed equation with the given resulting equation to solve for $m$.

We are given that the resulting curve is y=2x+18xy = 2x + \frac{1}{8x}. By comparing the terms with our deduced equation:
m22x=18x\frac{m^2}{2x} = \frac{1}{8x}
m2=28=14m^2 = \frac{2}{8} = \frac{1}{4}
Taking the square root:
m=±12m = \pm \frac{1}{2}
Since we are given 0<m<10 < m < 1 in the question stem, we reject the negative root to obtain:
m=12m = \frac{1}{2}

Answer

m=12m = \frac{1}{2}

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