曲线 CC 的参数方程为
x=2cos⁡t+1,y=sin⁡t,0≤t<2π.x = 2\cos t + 1, \quad y = \sin t , \quad 0 \le t < 2\pi .

  1. (i)

    画出 CC 的草图,标出其与 xx 轴交点的坐标。

  2. (ii)

    求当 CC 绕 xx 轴旋转 180∘180^\circ 时所得到的旋转体的体积的数值。

  3. (iii)

    求 CC 的直角坐标方程。

  4. (iv)

    描述一个能将曲线 CC 变换为以原点为圆心、半径为 2 的圆的变换序列。

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完整解法

9(i)

1. Find the coordinates of the intersections with the $x$-axis.

At the xx-axis, y=0y = 0.
sin⁡t=0\sin t = 0
Given 0≤t<2π0 \le t < 2\pi, t=0t = 0 or t=πt = \pi.

When t=0t = 0:
x=2cos⁡(0)+1=2(1)+1=3x = 2\cos(0) + 1 = 2(1) + 1 = 3
When t=πt = \pi:
x=2cos⁡(π)+1=2(−1)+1=−1x = 2\cos(\pi) + 1 = 2(-1) + 1 = -1
The coordinates of the intersections with the xx-axis are (−1,0)(-1, 0) and (3,0)(3, 0).

2. Determine the shape of the curve by finding its Cartesian equation to aid the sketch.

Rearrange the parametric equations to make the trigonometric functions the subject:
cos⁡t=x−12\cos t = \frac{x - 1}{2}
sin⁡t=y\sin t = y
Using the identity cos⁡2t+sin⁡2t=1\cos^2 t + \sin^2 t = 1:
(x−12)2+y2=1\left(\frac{x - 1}{2}\right)^2 + y^2 = 1
(x−1)24+y2=1\frac{(x - 1)^2}{4} + y^2 = 1
This represents an ellipse centred at (1,0)(1, 0). Because the domain is 0≤t<2π0 \le t < 2\pi, the graph is a complete ellipse.

3. Describe and produce the required sketch.

Draw a set of xx and yy axes.
Sketch a full oval (ellipse) centred at (1,0)(1, 0).
The curve intersects the xx-axis at (−1,0)(-1, 0) and (3,0)(3, 0).
The highest point on the curve is (1,1)(1, 1) and the lowest is (1,−1)(1, -1).

答案

A full ellipse centred at (1,0)(1, 0) with horizontal limits from x=−1x = -1 to x=3x = 3 and vertical limits from y=−1y = -1 to y=1y = 1. The xx-axis intersections are (−1,0)(-1, 0) and (3,0)(3, 0).

9(ii)

1. Set up the parametric integral for the volume of revolution.

Rotating the entire ellipse CC by 180∘180^\circ about the xx-axis generates the same complete solid (an ellipsoid) as rotating the upper half of CC by 360∘360^\circ.
The upper half of CC corresponds to y≥0y \ge 0, which implies 0≤t≤π0 \le t \le \pi.
The volume VV is given by the standard formula for a full revolution of the upper boundary:
V=π∫−13y2 dxV = \pi \int_{-1}^{3} y^2 \, \mathrm{d}x
Using the parametric equations x=2cos⁡t+1  ⟹  dxdt=−2sin⁡tx = 2\cos t + 1 \implies \frac{\mathrm{d}x}{\mathrm{d}t} = -2\sin t, and y=sin⁡ty = \sin t:
When x=−1x = -1, t=πt = \pi; when x=3x = 3, t=0t = 0.
V=π∫π0(sin⁡t)2(−2sin⁡t) dtV = \pi \int_{\pi}^{0} (\sin t)^2 (-2\sin t) \, \mathrm{d}t

2. Simplify and evaluate the integral to find the numerical volume.

V=2π∫0πsin⁡3t dtV = 2\pi \int_{0}^{\pi} \sin^3 t \, \mathrm{d}t
V=2π∫0πsin⁡t(1−cos⁡2t) dtV = 2\pi \int_{0}^{\pi} \sin t (1 - \cos^2 t) \, \mathrm{d}t
V=2π[−cos⁡t+13cos⁡3t]0πV = 2\pi \left[ -\cos t + \frac{1}{3}\cos^3 t \right]_0^\pi
V=2π[(−cos⁡π+13cos⁡3π)−(−cos⁡0+13cos⁡30)]V = 2\pi \left[ \left(-\cos\pi + \frac{1}{3}\cos^3\pi\right) - \left(-\cos 0 + \frac{1}{3}\cos^3 0\right) \right]
V=2π[(1−13)−(−1+13)]V = 2\pi \left[ \left(1 - \frac{1}{3}\right) - \left(-1 + \frac{1}{3}\right) \right]
V=2π(23+23)=8π3V = 2\pi \left( \frac{2}{3} + \frac{2}{3} \right) = \frac{8\pi}{3}
(Note: Evaluation can also be done directly using a graphing calculator to obtain ≈8.38\approx 8.38.)

答案

8π3\frac{8\pi}{3} (or 8.388.38 to 3 s.f.)

9(iii)

1. Make $\cos t$ and $\sin t$ the subjects of the parametric equations.

x=2cos⁡t+1  ⟹  cos⁡t=x−12x = 2\cos t + 1 \implies \cos t = \frac{x-1}{2}
y=sin⁡t  ⟹  sin⁡t=yy = \sin t \implies \sin t = y

2. Eliminate the parameter $t$ using the Pythagorean identity.

Since cos⁡2t+sin⁡2t=1\cos^2 t + \sin^2 t = 1,
(x−12)2+y2=1\left(\frac{x-1}{2}\right)^2 + y^2 = 1
(x−1)24+y2=1\frac{(x-1)^2}{4} + y^2 = 1

答案

(x−1)24+y2=1\frac{(x-1)^2}{4} + y^2 = 1

9(iv)

1. State the cartesian equations of curve $C$ and the target circle.

From part (iii), the cartesian equation of CC is (x−1)24+y2=1\frac{(x-1)^2}{4} + y^2 = 1.

The target is a circle with radius 22 centred at the origin, which has the equation x2+y2=4x^2 + y^2 = 4. Dividing by 44, this can be written as:
x24+y24=1\frac{x^2}{4} + \frac{y^2}{4} = 1

2. Determine the transformation to center the curve at the origin.

To transform (x−1)24+y2=1\frac{(x-1)^2}{4} + y^2 = 1 to x24+y2=1\frac{x^2}{4} + y^2 = 1, we replace xx with x+1x+1.
This corresponds to a translation of −1-1 unit in the xx-direction, which is a translation by the vector (−10)\begin{pmatrix} -1 \\ 0 \end{pmatrix}.

3. Determine the transformation to adjust the semi-minor axis to match the circle's radius.

To transform x24+y2=1\frac{x^2}{4} + y^2 = 1 into the target circle x24+y24=1\frac{x^2}{4} + \frac{y^2}{4} = 1, we replace yy with y2\frac{y}{2}.
This corresponds to a stretch (or scaling) parallel to the yy-axis by a scale factor of 22.

(Note: As the two transformations affect different axes, they can be performed in either order.)

答案
  1. Translation by (−10)\begin{pmatrix} -1 \\ 0 \end{pmatrix}.
  2. Stretch parallel to the yy-axis with a scale factor of 2.

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