The curve CC has parametric equations
x=2t−1t2,y=2t+1t,t∈R,t≠0.x = 2t - \frac{1}{t^2}, \quad y = 2t + \frac{1}{t}, \quad t \in \mathbb{R}, t \ne 0.
The point PP on the curve has parameter t=1t = 1.

  1. (i)

    Find the equation of tangent and normal to CC at the point PP.

  2. (ii)

    The tangent at PP meets the yy-axis at BB. The normal at PP meets the xx-axis at AA. If OO is the origin, find the area of the quadrilateral OAPBOAPB.

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Worked solution

3(i)

1. Find the coordinates of point $P$ by substituting $t=1$.

x=2(1)−112=1x = 2(1) - \frac{1}{1^2} = 1
y=2(1)+11=3y = 2(1) + \frac{1}{1} = 3
So, PP is (1,3)(1, 3).

2. Find the derivatives of $x$ and $y$ with respect to $t$.

x=2t−t−2  ⟹  dxdt=2+2t−3=2+2t3x = 2t - t^{-2} \implies \frac{dx}{dt} = 2 + 2t^{-3} = 2 + \frac{2}{t^3}
y=2t+t−1  ⟹  dydt=2−t−2=2−1t2y = 2t + t^{-1} \implies \frac{dy}{dt} = 2 - t^{-2} = 2 - \frac{1}{t^2}

3. Find the gradient of the tangent, $\frac{dy}{dx}$, at $t = 1$.

At t=1t = 1:
dxdt=2+2=4\frac{dx}{dt} = 2 + 2 = 4
dydt=2−1=1\frac{dy}{dt} = 2 - 1 = 1

dydx=dydtdxdt=14\frac{dy}{dx} = \frac{\frac{dy}{dt}}{\frac{dx}{dt}} = \frac{1}{4}

4. Find the equation of the tangent at $P$.

Using the point (1,3)(1, 3) and gradient m=14m = \frac{1}{4}:
y−3=14(x−1)y - 3 = \frac{1}{4}(x - 1)
4y−12=x−14y - 12 = x - 1
4y−x=114y - x = 11

5. Find the equation of the normal at $P$.

The gradient of the normal is −1m=−4-\frac{1}{m} = -4.
y−3=−4(x−1)y - 3 = -4(x - 1)
y−3=−4x+4y - 3 = -4x + 4
y+4x=7y + 4x = 7

Answer

Tangent: 4y−x=114y - x = 11
Normal: y+4x=7y + 4x = 7

3(ii)

1. Retrieve the coordinates of $P$ and the equations of the tangent and normal from part (i)

(Self-contained derivation for completeness)
At t=1t = 1, x=2(1)−112=1x = 2(1) - \frac{1}{1^2} = 1 and y=2(1)+11=3y = 2(1) + \frac{1}{1} = 3, giving P(1,3)P(1, 3).
dxdt=2+2t3=4\frac{dx}{dt} = 2 + \frac{2}{t^3} = 4 and dydt=2−1t2=1\frac{dy}{dt} = 2 - \frac{1}{t^2} = 1 at t=1t = 1, so the gradient of the tangent is dydx=14\frac{dy}{dx} = \frac{1}{4}.

Equation of tangent at PP:
y−3=14(x−1)  ⟹  y=14x+114y - 3 = \frac{1}{4}(x - 1) \implies y = \frac{1}{4}x + \frac{11}{4}

Equation of normal at PP:
y−3=−4(x−1)  ⟹  y=−4x+7y - 3 = -4(x - 1) \implies y = -4x + 7

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

The tangent meets the yy-axis at BB. Substituting x=0x = 0 into the tangent equation:
y=114  ⟹  B(0,114)y = \frac{11}{4} \implies B\left(0, \frac{11}{4}\right)

The normal meets the xx-axis at AA. Substituting y=0y = 0 into the normal equation:
0=−4x+7  ⟹  x=74  ⟹  A(74,0)0 = -4x + 7 \implies x = \frac{7}{4} \implies A\left(\frac{7}{4}, 0\right)

3. Calculate the area of the quadrilateral $OAPB$

We can split quadrilateral OAPBOAPB into two triangles: △OAP\triangle OAP and △OPB\triangle OPB.

Area of △OAP=12×base OA×perpendicular height of P\triangle OAP = \frac{1}{2} \times \text{base } OA \times \text{perpendicular height of } P from the xx-axis
=12(74)(3)=218= \frac{1}{2} \left(\frac{7}{4}\right) (3) = \frac{21}{8}

Area of △OPB=12×base OB×perpendicular height of P\triangle OPB = \frac{1}{2} \times \text{base } OB \times \text{perpendicular height of } P from the yy-axis
=12(114)(1)=118= \frac{1}{2} \left(\frac{11}{4}\right) (1) = \frac{11}{8}

Total Area =218+118=328=4= \frac{21}{8} + \frac{11}{8} = \frac{32}{8} = 4

Answer

44

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