HyperbolaMedium

Question

Let O be the origin, and P and Q be two points on the rectangular hyperbola $xy = 12$ such that the mid point of the line segment PQ is $\left( \frac{1}{2}, - \frac{1}{2} \right)$. Then the area of the triangle OPQ equals :

Options

A.$\frac{3}{2}$
B.$\frac{5}{2}$
C.$\frac{7}{2}$
D.$\frac{9}{2}$

Solution

Sol. [Key Concept: Chord with given midpoint ($T = S_{1}$) gives $P,Q$ on $xy = 12$; area of $OPQ$ by the shoelace formula.]

Equation of chord whose midpoint is $\left( \frac{1}{2},\frac{- 1}{2} \right)$

$$\begin{matrix} & \frac{x}{x_{1}} + \frac{y}{y_{1}} = 2 \\ & 2x - 2y = 2 \\ & x - y = 1 \end{matrix}$$

solving with $xy = 12$

$$P( - 3, - 4),Q(4,3)$$

Area $( \bigtriangleup OPQ) = \frac{1}{2}| - 3 \times 3 - ( - 4)(4)| = \frac{7}{2}$

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