The curve CC has equation y=x−2kx2+x−2y = \frac{x - 2}{kx^2 + x - 2}, where k>1k > 1.

  1. (i)

    Find the equation of the tangent at the point AA where CC cuts the yy-axis.

  2. (ii)

    Sketch CC, giving the equations of asymptotes, the coordinates of turning points and axial intercepts in terms of kk, if any.

  3. (iii)

    Find the equation of the normal at the point BB where CC cuts the xx-axis. Leave your answer in terms of kk.

  4. (iv)

    Hence show that the value of the area bounded by the tangent at AA, the normal at BB and both the xx- and yy-axes is more than 158\frac{15}{8} square units.

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Worked solution

7(i)

1. Find the coordinates of point $A$ where the curve cuts the $y$-axis.

Substitute x=0x = 0 into the equation of CC:
y=0−2k(0)2+0−2=−2−2=1y = \frac{0 - 2}{k(0)^2 + 0 - 2} = \frac{-2}{-2} = 1
Point AA is (0,1)(0, 1).

2. Find the gradient of the tangent at $A$ by finding $\frac{\mathrm{d}y}{\mathrm{d}x}$ and evaluating it at $x = 0$.

Using the quotient rule:
dydx=(kx2+x−2)ddx(x−2)−(x−2)ddx(kx2+x−2)(kx2+x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(kx^2 + x - 2)\frac{\mathrm{d}}{\mathrm{d}x}(x - 2) - (x - 2)\frac{\mathrm{d}}{\mathrm{d}x}(kx^2 + x - 2)}{(kx^2 + x - 2)^2}
dydx=(kx2+x−2)(1)−(x−2)(2kx+1)(kx2+x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(kx^2 + x - 2)(1) - (x - 2)(2kx + 1)}{(kx^2 + x - 2)^2}

Substitute x=0x = 0:
dydx∣x=0=(−2)(1)−(−2)(1)(−2)2=−2+24=0\left. \frac{\mathrm{d}y}{\mathrm{d}x} \right|_{x=0} = \frac{(-2)(1) - (-2)(1)}{(-2)^2} = \frac{-2 + 2}{4} = 0
The gradient of the tangent at AA is 00.

3. State the equation of the tangent at point $A$.

Since the gradient is 00 and the line passes through (0,1)(0, 1), the tangent is a horizontal line.
y−1=0(x−0)y - 1 = 0(x - 0)
y=1y = 1

Answer

y=1y = 1

7(ii)

1. Determine the axial intercepts of the curve.

To find the xx-intercept, set y=0y = 0:
x−2kx2+x−2=0  ⟹  x−2=0  ⟹  x=2\frac{x - 2}{kx^2 + x - 2} = 0 \implies x - 2 = 0 \implies x = 2
Thus, the xx-intercept is (2,0)(2, 0).

To find the yy-intercept, set x=0x = 0:
y=0−2k(0)2+0−2=−2−2=1y = \frac{0 - 2}{k(0)^2 + 0 - 2} = \frac{-2}{-2} = 1
Thus, the yy-intercept is (0,1)(0, 1).

2. Find the equations of the horizontal and vertical asymptotes.

For the horizontal asymptote, we consider the behavior as x→±∞x \to \pm\infty:
lim⁡x→±∞x−2kx2+x−2=lim⁡x→±∞1x−2x2k+1x−2x2=0\lim_{x \to \pm\infty} \frac{x - 2}{kx^2 + x - 2} = \lim_{x \to \pm\infty} \frac{\frac{1}{x} - \frac{2}{x^2}}{k + \frac{1}{x} - \frac{2}{x^2}} = 0
So, the horizontal asymptote is y=0y = 0.

For vertical asymptotes, the denominator approaches zero:
kx2+x−2=0kx^2 + x - 2 = 0
Using the quadratic formula to solve for xx:
x=−1±12−4(k)(−2)2k=−1±1+8k2kx = \frac{-1 \pm \sqrt{1^2 - 4(k)(-2)}}{2k} = \frac{-1 \pm \sqrt{1 + 8k}}{2k}
These are the equations of the vertical asymptotes.

3. Differentiate the equation to find the $x$-coordinates of the turning points.

Using the quotient rule vu′−uv′v2\frac{v u' - u v'}{v^2}:
dydx=(kx2+x−2)(1)−(x−2)(2kx+1)(kx2+x−2)2\frac{dy}{dx} = \frac{(kx^2 + x - 2)(1) - (x - 2)(2kx + 1)}{(kx^2 + x - 2)^2}
dydx=kx2+x−2−(2kx2+x−4kx−2)(kx2+x−2)2\frac{dy}{dx} = \frac{kx^2 + x - 2 - (2kx^2 + x - 4kx - 2)}{(kx^2 + x - 2)^2}
dydx=−kx2+4kx(kx2+x−2)2\frac{dy}{dx} = \frac{-kx^2 + 4kx}{(kx^2 + x - 2)^2}
For turning points, set dydx=0\frac{dy}{dx} = 0:
−kx2+4kx=0  ⟹  −kx(x−4)=0-kx^2 + 4kx = 0 \implies -kx(x - 4) = 0
Since k>1k > 1 (meaning k≠0k \neq 0), the possible solutions are:
x=0orx=4x = 0 \quad \text{or} \quad x = 4

4. Calculate the corresponding $y$-coordinates to finalize the turning points.

When x=0x = 0:
y=1y = 1
When x=4x = 4:
y=4−2k(42)+4−2=216k+2=18k+1y = \frac{4 - 2}{k(4^2) + 4 - 2} = \frac{2}{16k + 2} = \frac{1}{8k + 1}
So, the coordinates of the turning points are (0,1)(0, 1) and (4,18k+1)\left(4, \frac{1}{8k + 1}\right).

(The student would then synthesize these features to sketch the graph, with (0,1)(0, 1) as a local minimum and (4,18k+1)\left(4, \frac{1}{8k+1}\right) as a local maximum, properly bound by the respective asymptotes).

Answer

Axial Intercepts:
xx-intercept: (2,0)(2, 0)
yy-intercept: (0,1)(0, 1)

Equations of Asymptotes:
Horizontal asymptote: y=0y = 0
Vertical asymptotes: x=−1±1+8k2kx = \frac{-1 \pm \sqrt{1 + 8k}}{2k}

Turning Points:
(0,1)(0, 1) and (4,18k+1)\left(4, \frac{1}{8k+1}\right)

(The required sketch of CC should show the curve passing through these intercepts and turning points, guided by the stated asymptotes.)

7(iii)

1. Find the coordinates of point $B$ where the curve intersects the $x$-axis.

At the xx-axis, y=0y = 0.
x−2kx2+x−2=0\frac{x - 2}{kx^2 + x - 2} = 0
x−2=0  ⟹  x=2x - 2 = 0 \implies x = 2
The coordinates of BB are (2,0)(2, 0).

2. Differentiate $y$ with respect to $x$ to find the gradient function.

Using the quotient rule:
dydx=(kx2+x−2)ddx(x−2)−(x−2)ddx(kx2+x−2)(kx2+x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(kx^2 + x - 2)\frac{\mathrm{d}}{\mathrm{d}x}(x - 2) - (x - 2)\frac{\mathrm{d}}{\mathrm{d}x}(kx^2 + x - 2)}{(kx^2 + x - 2)^2}
dydx=(kx2+x−2)(1)−(x−2)(2kx+1)(kx2+x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(kx^2 + x - 2)(1) - (x - 2)(2kx + 1)}{(kx^2 + x - 2)^2}

3. Evaluate the gradient of the normal at $B$ and find its equation.

Substitute x=2x = 2 into the derivative to find the gradient of the tangent, mtm_t:
mt=(k(2)2+2−2)(1)−(0)(2k(2)+1)(k(2)2+2−2)2m_t = \frac{(k(2)^2 + 2 - 2)(1) - (0)(2k(2) + 1)}{(k(2)^2 + 2 - 2)^2}
mt=4k(4k)2=14km_t = \frac{4k}{(4k)^2} = \frac{1}{4k}
The gradient of the normal, mnm_n, is −1mt-\frac{1}{m_t}:
mn=−4km_n = -4k
Using the point-gradient form for B(2,0)B(2, 0):
y−0=−4k(x−2)y - 0 = -4k(x - 2)
y=−4kx+8ky = -4kx + 8k

Answer

y=−4kx+8ky = -4kx + 8k

7(iv)

1. State or determine the equations of the tangent at $A$ and the normal at $B$ from earlier parts.

From earlier parts, CC cuts the yy-axis at A(0,1)A(0,1) and the xx-axis at B(2,0)B(2,0).
dydx=(kx2+x−2)(1)−(x−2)(2kx+1)(kx2+x−2)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(kx^2+x-2)(1) - (x-2)(2kx+1)}{(kx^2+x-2)^2}
At A(0,1)A(0,1), dydx=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0, so the equation of the tangent is y=1y = 1.
At B(2,0)B(2,0), dydx=1(4k)−0(4k)2=14k\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{1(4k) - 0}{(4k)^2} = \frac{1}{4k}. The gradient of the normal is −4k-4k.
Equation of the normal at BB: y−0=−4k(x−2)  ⟹  y=−4kx+8ky - 0 = -4k(x - 2) \implies y = -4kx + 8k.

2. Find the point of intersection between the tangent at $A$ and the normal at $B$.

Substitute y=1y = 1 into the normal equation:
1=−4kx+8k1 = -4kx + 8k
4kx=8k−1  ⟹  x=8k−14k=2−14k4kx = 8k - 1 \implies x = \frac{8k-1}{4k} = 2 - \frac{1}{4k}
The intersection point is (2−14k,1)\left(2 - \frac{1}{4k}, 1\right).

3. Calculate the area of the region bounded by the tangent, the normal, and both axes.

The region is bounded by y=1y=1, y=−4kx+8ky=-4kx+8k, y=0y=0 (xx-axis), and x=0x=0 (yy-axis).
This forms a right-angled trapezium with vertices (0,0)(0,0), (2,0)(2,0), (2−14k,1)\left(2 - \frac{1}{4k}, 1\right), and (0,1)(0,1).
The parallel horizontal sides lie on y=0y=0 and y=1y=1 with lengths 22 and 2−14k2 - \frac{1}{4k} respectively. The height along the yy-axis is 11.
Area=12×height×(sum of parallel sides)\text{Area} = \frac{1}{2} \times \text{height} \times (\text{sum of parallel sides})
Area=12(1)[2+(2−14k)]=12(4−14k)=2−18k\text{Area} = \frac{1}{2} (1) \left[ 2 + \left(2 - \frac{1}{4k}\right) \right] = \frac{1}{2} \left(4 - \frac{1}{4k}\right) = 2 - \frac{1}{8k}

4. Use the given condition $k > 1$ to show that the area is strictly greater than $\frac{15}{8}$.

We are given that k>1k > 1.
8k>8  ⟹  18k<188k > 8 \implies \frac{1}{8k} < \frac{1}{8}
Multiplying by −1-1 reverses the inequality:
−18k>−18-\frac{1}{8k} > -\frac{1}{8}
Therefore, adding 22 to both sides:
Area=2−18k>2−18=158\text{Area} = 2 - \frac{1}{8k} > 2 - \frac{1}{8} = \frac{15}{8}
Thus, the area is more than 158\frac{15}{8} square units.

Answer

Area =2−18k= 2 - \frac{1}{8k}. Since k>1k > 1, Area >158> \frac{15}{8} square units. (Shown)

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