曲线 的方程为 。
- (a)
画出 的草图,指出任何极值点以及与坐标轴交点的坐标,并写出所有渐近线的方程。
- (b)
确定 的确切值集合,使得对于所有实数 , 与直线 都有两个交点。
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完整解法
2(a)
1. Find the point(s) of intersection with the coordinate axes.
Substitute into the equation of :
The -intercept is .
Substitute :
The discriminant .
Since , there are no real roots, hence the curve does not intersect the -axis.
2. Determine the equations of the asymptotes.
The denominator cannot be zero, giving the vertical asymptote:
To find the oblique asymptote, perform polynomial division on :
As , , so the oblique asymptote is:
3. Find the coordinates of the turning points.
Differentiate with respect to :
At the turning points, :
Substitute back to find the corresponding -values:
When , .
When , .
The turning points are and .
4. Describe the sketch of the curve $C$.
Draw a set of axes and include the following:
- A vertical dashed line at .
- A dashed line for the oblique asymptote .
- The upper branch for , passing through the local minimum and -intercept at , bounded by the asymptotes.
- The lower branch for , with a local maximum at , bounded by the asymptotes.
Turning points: and
Intersection with axes: only
Asymptotes: and
(A complete sketch should be drawn on the coordinate axes incorporating these features.)
2(b)
1. Form a quadratic equation in $x$ for the intersection
At intersection points .
Multiply by :
Rearranging:
For two points of intersection this quadratic must have two distinct real roots, so we also need .
2. Apply discriminant > 0 and solve the quadratic inequality in $k$
Two distinct real roots require discriminant :
Roots of : .
Since the parabola opens upward:
3. Exclude the degenerate value and state the set
When , i.e. , the equation becomes linear , giving only ONE intersection. Since , lies in the region above and must be removed.
Hence
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