已知曲线 的方程为 ,其中 且 。
- (i)
不使用计算器,求 不能取的值的集合。
- (ii)
画出 的草图,明确指出所有渐近线的方程、驻点的坐标以及曲线与坐标轴的交点。
- (iii)
推导 的取值范围,使得方程
没有根,其中 是正常数。
- (iv)
完整地描述一系列变换,这些变换将曲线 变换为曲线 。
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完整解法
10(i)
1. Rearrange the given equation to form a quadratic equation in $x$.
2. Apply the discriminant condition for real roots to establish an inequality in $y$.
For to have real values, the discriminant must be non-negative ():
3. Expand and simplify the discriminant inequality.
4. Factorise and solve for the valid range of $y$, then state the set of values $y$ cannot take.
Hence, the possible values for are or .
The set of values that cannot take is the complement of this range:
10(ii)
1. Determine the equations of the asymptotes.
By algebraic division, rewrite the rational function:
The vertical asymptote occurs when the denominator is zero:
As , the fractional term , giving the oblique asymptote:
2. Find the coordinates of the stationary points.
Differentiate with respect to to find the stationary points:
Set :
Substitute the -values back into the equation of :
When , .
When , .
The stationary points are and .
(Note: Students can also deduce the -values and from the boundary values established in part (i), then solve for the corresponding -values.)
3. Find the point(s) where the curve crosses the axes.
Substitute to find the -intercept:
The curve crosses the -axis at .
Substitute to find any -intercepts:
Check the discriminant :
Since , there are no real solutions, hence the curve does not cross the -axis.
4. Sketch the curve $C$.
Draw the coordinate axes and plot the key features:
- Draw and label the dashed lines for the asymptotes and .
- Plot the local maximum point at and the local minimum point at .
- Plot the -intercept at .
- Sketch the two continuous branches of the rational function approaching the asymptotes smoothly.
Asymptotes:
Stationary points: and
Axial intercept:
(A sketch of the curve showing the two branches, bounded by the stated asymptotes, with the turning points and -intercept clearly marked.)
10(iii)
1. Apply a substitution to simplify the given equation.
Let . Then .
By completing the square for the expression :
Substitute these into the given equation:
2. Expand and simplify the equation into a polynomial in $X$.
Expand the terms:
Group the terms by powers of :
3. Analyze the roots of the quadratic in $X^2$ to find the condition for no real roots.
Let . Since , we require for to be real.
The equation is a quadratic in :
For the original equation to have no root, this quadratic must have no positive real root for . Notice that since , the coefficients and are both strictly negative.
- If , the signs of the coefficients are , meaning the product of roots is negative, so there is exactly one positive real root.
- If , all coefficients are . For any , the polynomial's value is strictly negative and can never equal , so there are no positive real roots.
Thus, for no root, we must have:
Since is a positive constant, .
10(iv)
1. Express the given rational function in quotient-remainder form.
By algebraic manipulation or polynomial long division:
2. Relate the simplified expression to the original function.
Let . We want to express the simplified in terms of .
Rewrite by completing the form of :
3. State the sequence of transformations.
The equation can be mapped from by:
- A reflection in the -axis, transforming into .
- A translation by the vector , transforming into .
- Reflection in the -axis.
- Translation by the vector .
(Note: Other equivalent sequences of transformations are acceptable)
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