The curve C has parametric equations $x = 2t - \dfrac{1}{t^{2}},\quad y = 2t + \dfrac{1}{t},\quad t \in \mathbb{R},\ t \ne 0. The point P on the curve has parameter t=1t = 1.

  1. (i)

    Find the equation of tangent and normal to C at the point P.

  2. (ii)

    The tangent at P meets the y-axis at B. The normal at P meets the x-axis at A. If O is the origin, find the area of the quadrilateral OAPB.

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Worked solution

3(i)

1. Find the coordinates of $P$ by substituting $t = 1$ into the parametric equations.

x=2(1)−112=2−1=1x = 2(1) - \frac{1}{1^2} = 2 - 1 = 1
y=2(1)+11=2+1=3y = 2(1) + \frac{1}{1} = 2 + 1 = 3
The coordinates of PP are (1,3)(1, 3).

2. Differentiate $x$ and $y$ with respect to $t$ and evaluate them at $t = 1$.

x=2t−t−2  ⟹  dxdt=2+2t−3x = 2t - t^{-2} \implies \frac{dx}{dt} = 2 + 2t^{-3}
At t=1t = 1, dxdt=2+2(1)=4\frac{dx}{dt} = 2 + 2(1) = 4

y=2t+t−1  ⟹  dydt=2−t−2y = 2t + t^{-1} \implies \frac{dy}{dt} = 2 - t^{-2}
At t=1t = 1, dydt=2−1=1\frac{dy}{dt} = 2 - 1 = 1

3. Calculate the gradient of the tangent, $\frac{dy}{dx}$, at point $P$.

Using the chain rule for parametric equations:
dydx=(dydt)(dxdt)=14\frac{dy}{dx} = \frac{\left(\frac{dy}{dt}\right)}{\left(\frac{dx}{dt}\right)} = \frac{1}{4}

The gradient of the tangent at PP is 14\frac{1}{4}.

4. Form the equations of the tangent and the normal.

Equation of the tangent (gradient m=14m = \frac{1}{4}):
y−3=14(x−1)y - 3 = \frac{1}{4}(x - 1)
4y−12=x−1  ⟹  4y=x+11 or y=14x+1144y - 12 = x - 1 \implies 4y = x + 11 \text{ or } y = \frac{1}{4}x + \frac{11}{4}

Gradient of the normal is mn=−114=−4m_n = -\frac{1}{\frac{1}{4}} = -4.
Equation of the normal:
y−3=−4(x−1)y - 3 = -4(x - 1)
y−3=−4x+4  ⟹  y=−4x+7y - 3 = -4x + 4 \implies y = -4x + 7

Answer

Tangent: y=14x+114y = \frac{1}{4}x + \frac{11}{4}, Normal: y=−4x+7y = -4x + 7

3(ii)

1. Find the coordinates of P and the gradient of the curve at $t=1$ (derived from part i).

x=2t−1t2,y=2t+1tx = 2t - \frac{1}{t^{2}}, \quad y = 2t + \frac{1}{t}
At t=1t=1, x=2(1)−1=1x = 2(1) - 1 = 1 and y=2(1)+1=3y = 2(1) + 1 = 3. Thus PP is (1,3)(1, 3).
dxdt=2+2t3,dydt=2−1t2\frac{dx}{dt} = 2 + \frac{2}{t^3}, \quad \frac{dy}{dt} = 2 - \frac{1}{t^2}
At t=1t=1, dxdt=4\frac{dx}{dt} = 4 and dydt=1\frac{dy}{dt} = 1.
dydx=dydtdxdt=14\frac{dy}{dx} = \frac{\frac{dy}{dt}}{\frac{dx}{dt}} = \frac{1}{4}

2. Form equations of the tangent and normal to find the coordinates of points A and B (derived from part i).

Equation of the tangent at P(1,3)P(1,3):
y−3=14(x−1)  ⟹  y=14x+114y - 3 = \frac{1}{4}(x - 1) \implies y = \frac{1}{4}x + \frac{11}{4}
The tangent meets the yy-axis (x=0x=0) at BB, so y=114y = \frac{11}{4}. Thus, B(0,114)B\left(0, \frac{11}{4}\right).

Equation of the normal at P(1,3)P(1,3):
y−3=−4(x−1)  ⟹  y=−4x+7y - 3 = -4(x - 1) \implies y = -4x + 7
The normal meets the xx-axis (y=0y=0) at AA, so 0=−4x+7  ⟹  x=740 = -4x + 7 \implies x = \frac{7}{4}. Thus, A(74,0)A\left(\frac{7}{4}, 0\right).

3. Calculate the area of quadrilateral OAPB.

The coordinates are O(0,0)O(0,0), A(74,0)A\left(\frac{7}{4}, 0\right), P(1,3)P(1, 3), and B(0,114)B\left(0, \frac{11}{4}\right).
Using the shoelace (array) method to find the area of the polygon:
Area=12∣0741000031140∣\text{Area} = \frac{1}{2} \left| \begin{matrix} 0 & \frac{7}{4} & 1 & 0 & 0 \\ 0 & 0 & 3 & \frac{11}{4} & 0 \end{matrix} \right|
Area=12∣(0⋅0+74⋅3+1⋅114+0⋅0)−(0⋅74+0⋅1+3⋅0+114⋅0)∣\text{Area} = \frac{1}{2} \left| \left( 0 \cdot 0 + \frac{7}{4} \cdot 3 + 1 \cdot \frac{11}{4} + 0 \cdot 0 \right) - \left( 0 \cdot \frac{7}{4} + 0 \cdot 1 + 3 \cdot 0 + \frac{11}{4} \cdot 0 \right) \right|
Area=12∣214+114∣\text{Area} = \frac{1}{2} \left| \frac{21}{4} + \frac{11}{4} \right|
Area=12(8)=4\text{Area} = \frac{1}{2}(8) = 4

Answer

4

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