[Cambridge/9709/2010]
The parametric equations of a curve are
x=1+ln⁡(t−2),y=t+9t,for t>2.x = 1 + \ln(t - 2), \quad y = t + \frac{9}{t}, \quad \text{for } t > 2.

  1. (i)

    Show that dydx=(t2−9)(t−2)t2\frac{\text{d}y}{\text{d}x} = \frac{(t^2 - 9)(t - 2)}{t^2}.

  2. (ii)

    Find the coordinates of the only point on the curve at which the gradient is equal to 0.

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Worked solution

Q7(i)

1. Differentiate $x$ with respect to $t$.

dxdt=1t−2\frac{\text{d}x}{\text{d}t} = \frac{1}{t-2}

2. Differentiate $y$ with respect to $t$ and combine into a single fraction.

y=t+9t−1y = t + 9t^{-1}
dydt=1−9t−2=1−9t2=t2−9t2\frac{\text{d}y}{\text{d}t} = 1 - 9t^{-2} = 1 - \frac{9}{t^2} = \frac{t^2 - 9}{t^2}

3. Apply the chain rule for parametric differentiation to find $\frac{\text{d}y}{\text{d}x}$.

dydx=dydtdxdt\frac{\text{d}y}{\text{d}x} = \frac{\frac{\text{d}y}{\text{d}t}}{\frac{\text{d}x}{\text{d}t}}
dydx=t2−9t21t−2\frac{\text{d}y}{\text{d}x} = \frac{\frac{t^2 - 9}{t^2}}{\frac{1}{t-2}}
dydx=(t2−9)(t−2)t2\frac{\text{d}y}{\text{d}x} = \frac{(t^2 - 9)(t - 2)}{t^2}

Answer

dydx=(t2−9)(t−2)t2\frac{\text{d}y}{\text{d}x} = \frac{(t^2 - 9)(t - 2)}{t^2} (Shown)

Q7(ii)

1. Equate the gradient to zero and solve for $t$, rejecting invalid values based on the given domain.

dydx=0\frac{\text{d}y}{\text{d}x} = 0
(t2−9)(t−2)t2=0\frac{(t^2 - 9)(t - 2)}{t^2} = 0
(t2−9)(t−2)=0(t^2 - 9)(t - 2) = 0
(t−3)(t+3)(t−2)=0(t - 3)(t + 3)(t - 2) = 0
Since the curve is only defined for t>2t > 2, we discard t=−3t = -3 and t=2t = 2.
The only valid solution is t=3t = 3.

2. Substitute $t = 3$ into the parametric equations to find the $x$ and $y$ coordinates.

Substitute t=3t = 3 into the equation for xx:
x=1+ln⁡(3−2)x = 1 + \ln(3 - 2)
x=1+ln⁡(1)x = 1 + \ln(1)
x=1+0=1x = 1 + 0 = 1

Substitute t=3t = 3 into the equation for yy:
y=3+93y = 3 + \frac{9}{3}
y=3+3=6y = 3 + 3 = 6

The coordinates of the point are (1,6)(1, 6).

Answer

(1,6)(1, 6)

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