A curve y=f(x)y=f(x) undergoes in succession, the following transformations: A: A reflection in the y-axis B: A translation of 2 units in the direction of the x-axis C: A translation of −a-a units in the direction of the y-axis, where a>1a>1

  1. (i)

    The equation of the resulting curve is y=g(x)y=g(x), where g(x)=1−3a−x+5g(x)=\dfrac{1-3a}{-x+5}. Determine the equation of the curve y=f(x)y=f(x), in terms of aa and xx.

  2. (ii)

    Sketch, on the same diagram, the graphs of y=g(x)y=g(x) and y=g−1(x)y=g^{-1}(x), indicating clearly the equations of the asymptotes and the axial intercepts.

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Worked solution

5(i)

1. Determine the expression for the curve after applying transformations A, B, and C sequentially to $f(x)$.

Let the initial curve be y=f(x)y = f(x).
Transformation A (reflection in the yy-axis): Replace xx with −x-x to get y=f(−x)y = f(-x).
Transformation B (translation of 22 units in the positive xx-direction): Replace xx with (x−2)(x-2) to get y=f(−(x−2))=f(2−x)y = f(-(x-2)) = f(2-x).
Transformation C (translation of −a-a units in the yy-direction): Subtract aa from the function to get y=f(2−x)−ay = f(2-x) - a.

2. Equate the resulting expression to $g(x)$ and isolate the function term.

Since the resulting curve is y=g(x)y = g(x), we have:
f(2−x)−a=1−3a−x+5f(2-x) - a = \frac{1-3a}{-x+5}
f(2−x)=1−3a−x+5+af(2-x) = \frac{1-3a}{-x+5} + a

3. Use substitution to find the original function $f(x)$ and simplify.

Let u=2−xu = 2-x, which implies x=2−ux = 2-u.
Substitute x=2−ux = 2-u into the equation:
f(u)=1−3a−(2−u)+5+af(u) = \frac{1-3a}{-(2-u)+5} + a
f(u)=1−3au+3+af(u) = \frac{1-3a}{u+3} + a
Combine into a single fraction over a common denominator:
f(u)=1−3a+a(u+3)u+3f(u) = \frac{1-3a + a(u+3)}{u+3}
f(u)=1−3a+au+3au+3f(u) = \frac{1-3a + au + 3a}{u+3}
f(u)=au+1u+3f(u) = \frac{au+1}{u+3}
Therefore, replacing uu with xx yields f(x)=ax+1x+3f(x) = \frac{ax+1}{x+3}.

Answer

f(x)=ax+1x+3f(x) = \dfrac{ax+1}{x+3}

5(ii)

1. Determine the asymptotes, intercepts, and general shape of $y = g(x)$.

Given g(x)=1−3a−x+5=3a−1x−5g(x) = \frac{1-3a}{-x+5} = \frac{3a-1}{x-5} and a>1a > 1.
Since a>1a > 1, the constant numerator 3a−1>03a-1 > 0.
The vertical asymptote occurs where the denominator is zero: x=5x = 5.
The horizontal asymptote is y=0y = 0 (as x→±∞x \to \pm \infty, g(x)→0g(x) \to 0).
To find the yy-intercept, substitute x=0x = 0: y=1−3a5y = \frac{1-3a}{5}.
Since a>1a > 1, 1−3a5<0\frac{1-3a}{5} < 0, so the yy-intercept is negative.
There is no xx-intercept since the numerator 1−3a≠01-3a \neq 0.
The curve is a rectangular hyperbola with two branches: one in the region x>5,y>0x > 5, y > 0 and the other in x<5,y<0x < 5, y < 0.

2. Determine the asymptotes and intercepts of $y = g^{-1}(x)$ using its reflection property.

The graph of y=g−1(x)y = g^{-1}(x) is the reflection of the graph of y=g(x)y = g(x) in the line y=xy = x.
Reflecting the asymptotes of g(x)g(x) gives the asymptotes of g−1(x)g^{-1}(x):

  • The vertical asymptote is x=0x = 0 (the yy-axis).
  • The horizontal asymptote is y=5y = 5.
    Reflecting the yy-intercept of g(x)g(x) gives the xx-intercept of g−1(x)g^{-1}(x), which is at x=1−3a5x = \frac{1-3a}{5}.
    Similarly, since g(x)g(x) has no xx-intercept, g−1(x)g^{-1}(x) has no yy-intercept.
    The curve is a rectangular hyperbola with two branches: one in the region x>0,y>5x > 0, y > 5 and the other in x<0,y<5x < 0, y < 5.

3. State the sketching characteristics for both graphs.

Sketch construction steps:

  1. Draw the xx and yy axes and the dashed lines for the asymptotes x=5x = 5 and y=5y = 5.
  2. For y=g(x)y = g(x), draw the two branches approaching x=5x=5 and y=0y=0, ensuring it crosses the yy-axis at (0,1−3a5)(0, \frac{1-3a}{5}).
  3. For y=g−1(x)y = g^{-1}(x), draw the two branches approaching x=0x=0 and y=5y=5, ensuring it crosses the xx-axis at (1−3a5,0)(\frac{1-3a}{5}, 0).
  4. Ensure the graphs are symmetrical about the line y=xy = x, and that they intersect each other exactly on this line (one intersection in the positive quadrant, one in the negative quadrant).
Answer

For y=g(x)y = g(x):

  • Asymptotes: x=5x = 5, y=0y = 0
  • Axial intercept: yy-intercept at (0,1−3a5)(0, \frac{1-3a}{5})

For y=g−1(x)y = g^{-1}(x):

  • Asymptotes: x=0x = 0, y=5y = 5
  • Axial intercept: xx-intercept at (1−3a5,0)(\frac{1-3a}{5}, 0)

(Both graphs should be sketched symmetrically about the line y=xy=x, showing their respective rectangular hyperbola branches approaching these asymptotes.)

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