曲线 的参数方程为
- (i)
画出 的草图,标出其与 轴交点的坐标。
- (ii)
求当 绕 轴旋转 时所得到的旋转体的体积的数值。
- (iii)
求 的直角坐标方程。
- (iv)
描述一个能将曲线 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 -axis, .
Given , or .
When :
When :
The coordinates of the intersections with the -axis are and .
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:
Using the identity :
This represents an ellipse centred at . Because the domain is , the graph is a complete ellipse.
3. Describe and produce the required sketch.
Draw a set of and axes.
Sketch a full oval (ellipse) centred at .
The curve intersects the -axis at and .
The highest point on the curve is and the lowest is .
A full ellipse centred at with horizontal limits from to and vertical limits from to . The -axis intersections are and .
9(ii)
1. Set up the parametric integral for the volume of revolution.
Rotating the entire ellipse by about the -axis generates the same complete solid (an ellipsoid) as rotating the upper half of by .
The upper half of corresponds to , which implies .
The volume is given by the standard formula for a full revolution of the upper boundary:
Using the parametric equations , and :
When , ; when , .
2. Simplify and evaluate the integral to find the numerical volume.
(Note: Evaluation can also be done directly using a graphing calculator to obtain .)
(or to 3 s.f.)
9(iii)
1. Make $\cos t$ and $\sin t$ the subjects of the parametric equations.
2. Eliminate the parameter $t$ using the Pythagorean identity.
Since ,
9(iv)
1. State the cartesian equations of curve $C$ and the target circle.
From part (iii), the cartesian equation of is .
The target is a circle with radius centred at the origin, which has the equation . Dividing by , this can be written as:
2. Determine the transformation to center the curve at the origin.
To transform to , we replace with .
This corresponds to a translation of unit in the -direction, which is a translation by the vector .
3. Determine the transformation to adjust the semi-minor axis to match the circle's radius.
To transform into the target circle , we replace with .
This corresponds to a stretch (or scaling) parallel to the -axis by a scale factor of .
(Note: As the two transformations affect different axes, they can be performed in either order.)
- Translation by .
- Stretch parallel to the -axis with a scale factor of 2.
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