A curve has parametric equations x=2tx=2t, y=e−t2y=e^{-t^2}.

  1. (a)

    Sketch the curve, stating the exact coordinates of any points of intersection with the axes and the equations of any asymptotes.

  2. (b)

    Find the equation of the normal to the curve at the point (2p,e−p2)\left(2p,e^{-p^2}\right).

  3. (c)

    The normal at the point C(2,1e)C\left(2,\frac{1}{e}\right) meets the x-axis at AA and the y-axis at BB. Find the value of the ratio ∣OA∣:∣OB∣|OA|:|OB|, where OO is the origin, giving your answer in terms of ee.

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Worked solution

7(a)

1. Find the coordinates of the intersection with the axes.

For the y-intercept, set x=0x = 0:
2t=0  ⟹  t=02t = 0 \implies t = 0
Substitute t=0t = 0 to find yy:
y=e−(0)2=e0=1y = e^{-(0)^2} = e^0 = 1
The point of intersection with the y-axis is (0,1)(0, 1).

For the x-intercept, set y=0y = 0:
e−t2=0e^{-t^2} = 0
Since e−t2>0e^{-t^2} > 0 for all real tt, there are no real solutions. Thus, the curve does not intersect the x-axis.

2. Determine the equations of any asymptotes.

Consider the behavior of the curve as tt becomes very large in magnitude (t→±∞t \to \pm\infty):
As t→±∞t \to \pm\infty, the x-coordinate x=2t→±∞x = 2t \to \pm\infty.
At the same time, the y-coordinate y=e−t2→0y = e^{-t^2} \to 0.
Therefore, the line y=0y = 0 is a horizontal asymptote.

3. Sketch the curve based on its properties (or via its Cartesian equation).

To confirm the shape, we can find the Cartesian equation by substituting t=x2t = \frac{x}{2}:
y=e−(x2)2=e−x24y = e^{-\left(\frac{x}{2}\right)^2} = e^{-\frac{x^2}{4}}
The graph is an even function (symmetric about the y-axis), passing through the maximum point (0,1)(0, 1) and forming a bell shape that approaches y=0y = 0 as x→±∞x \to \pm\infty.

(The student draws this bell-shaped curve on the axes, labelling the intercept (0,1)(0,1) and the asymptote y=0y=0.)

Answer

Intersection with axes: (0,1)(0, 1)
Asymptote: y=0y = 0

(The sketch is a bell-shaped curve with a maximum point at (0,1)(0,1) on the y-axis, symmetric about the y-axis, and asymptotic to the x-axis y=0y=0 in both the positive and negative x directions.)

7(b)

1. Differentiate parametrically to find the gradient of the tangent at $t = p$.

dxdt=2\frac{\mathrm{d}x}{\mathrm{d}t} = 2

dydt=−2te−t2\frac{\mathrm{d}y}{\mathrm{d}t} = -2te^{-t^2}

dydx=dydtdxdt=−2te−t22=−te−t2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{\frac{\mathrm{d}y}{\mathrm{d}t}}{\frac{\mathrm{d}x}{\mathrm{d}t}} = \frac{-2te^{-t^2}}{2} = -te^{-t^2}

At the point (2p,e−p2)(2p, e^{-p^2}), the parameter is t=pt = p.
So the gradient of the tangent is −pe−p2-pe^{-p^2}.

2. Find the gradient of the normal and formulate the equation of the normal.

The gradient of the normal is mn=−1−pe−p2=1pe−p2=ep2pm_n = -\frac{1}{-pe^{-p^2}} = \frac{1}{pe^{-p^2}} = \frac{e^{p^2}}{p}.

Using the point-slope form, the equation of the normal at (2p,e−p2)(2p, e^{-p^2}) is:
y−y1=mn(x−x1)y - y_1 = m_n(x - x_1)

y−e−p2=ep2p(x−2p)y - e^{-p^2} = \frac{e^{p^2}}{p}(x - 2p)

y−e−p2=ep2px−2ep2y - e^{-p^2} = \frac{e^{p^2}}{p}x - 2e^{p^2}

y=ep2px−2ep2+e−p2y = \frac{e^{p^2}}{p}x - 2e^{p^2} + e^{-p^2}

Answer

y=ep2px−2ep2+e−p2y = \frac{e^{p^2}}{p}x - 2e^{p^2} + e^{-p^2}

7(c)

1. Find the parameter $t$ and the equation of the normal at point $C$.

At point C(2,1e)C\left(2, \frac{1}{e}\right), we use the x-coordinate to find tt:
x=2t  ⟹  2=2t  ⟹  t=1x = 2t \implies 2 = 2t \implies t=1
Using the derivatives from the parametric equations:
dxdt=2anddydt=−2te−t2\frac{dx}{dt} = 2 \quad \text{and} \quad \frac{dy}{dt} = -2te^{-t^2}
dydx=dy/dtdx/dt=−2te−t22=−te−t2\frac{dy}{dx} = \frac{dy/dt}{dx/dt} = \frac{-2te^{-t^2}}{2} = -te^{-t^2}
At t=1t=1, the gradient of the tangent is:
mt=−1(e−1)=−1em_t = -1(e^{-1}) = -\frac{1}{e}
The gradient of the normal mnm_n is the negative reciprocal of the tangent gradient:
mn=−1mt=em_n = -\frac{1}{m_t} = e
Using the point-slope form, the equation of the normal at CC is:
y−1e=e(x−2)y - \frac{1}{e} = e(x - 2)

2. Find the coordinates of intersection points $A$ and $B$.

The normal meets the x-axis at AA. Set y=0y = 0:
0−1e=e(x−2)0 - \frac{1}{e} = e(x - 2)
x−2=−1e2  ⟹  x=2−1e2x - 2 = -\frac{1}{e^2} \implies x = 2 - \frac{1}{e^2}
Thus, AA is (2−1e2,0)\left(2 - \frac{1}{e^2}, 0\right).

The normal meets the y-axis at BB. Set x=0x = 0:
y−1e=e(0−2)y - \frac{1}{e} = e(0 - 2)
y=1e−2ey = \frac{1}{e} - 2e
Thus, BB is (0,1e−2e)\left(0, \frac{1}{e} - 2e\right).

3. Calculate the distances $|OA|$ and $|OB|$ to deduce the ratio.

The distance ∣OA∣|OA| is the magnitude of the x-coordinate of AA:
∣OA∣=∣2−1e2∣=2−1e2(since 2>1e2)|OA| = \left| 2 - \frac{1}{e^2} \right| = 2 - \frac{1}{e^2} \quad (\text{since } 2 > \frac{1}{e^2})
The distance ∣OB∣|OB| is the magnitude of the y-coordinate of BB:
∣OB∣=∣1e−2e∣=2e−1e(since 2e>1e)|OB| = \left| \frac{1}{e} - 2e \right| = 2e - \frac{1}{e} \quad (\text{since } 2e > \frac{1}{e})
We can factor out ee from ∣OB∣|OB| to compare with ∣OA∣|OA|:
∣OB∣=e(2−1e2)|OB| = e\left(2 - \frac{1}{e^2}\right)
The required ratio is:
∣OA∣:∣OB∣=(2−1e2):e(2−1e2)|OA| : |OB| = \left(2 - \frac{1}{e^2}\right) : e\left(2 - \frac{1}{e^2}\right)
∣OA∣:∣OB∣=1:e|OA| : |OB| = 1 : e

Answer

1:e1 : e

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