曲线 CC 的方程为 y=x−2kx2+x−2y = \frac{x - 2}{kx^2 + x - 2},其中 k>1k > 1。

  1. (i)

    求曲线 CC 与 yy 轴交点 AA 处的切线方程。

  2. (ii)

    画出 CC 的草图,给出渐近线方程、转折点坐标以及与坐标轴的截距(如果有的话,均用 kk 表示)。

  3. (iii)

    求 CC 与 xx 轴相交的点 BB 处的法线方程。答案用 kk 表示。

  4. (iv)

    由此证明,由在 AA 点的切线、在 BB 点的法线以及 xx 轴和 yy 轴所围成的区域面积大于 158\frac{15}{8} 平方单位。

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完整解法

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

答案

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).

答案

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

答案

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.

答案

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

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