Question

# The tangent at the point $$P$$ on the rectangular hyperbola $$xy={k}^{2}$$ with $$C$$ intersects the coordinate axes at $$Q$$ and $$R$$ . Locus of the circumcentre of triangle $$CQR$$ is

A
x2+y2=2k2
B
x2+y2=k2
C
xy=k2
D
none of these

Solution

## The correct option is C $$xy={k}^{2}$$Given:A tangent at the point $$P$$ on the rectangular hyperbola $$xy={k}^{2}$$ with $$C$$ intersects the coordinate axes at $$Q$$ and $$R$$ where $$C\left(0,0\right)$$ is the center of hyperbola.$$\therefore\,\triangle{CQR}$$ is a rightangled triangle where $$\angle{C}={90}^{\circ}$$The circumcentre of a right angled triangle is the mid-point of its hypotenuse.Coordinates of hyperbola $$xy={k}^{2}$$ is $$\left(kt,\dfrac{k}{t}\right)$$The tangent at $$\left(kt,\dfrac{k}{t}\right)$$ is $$x+y{t}^{2}=2kt$$cuts the $$y-$$ axis at $$Q$$At $$x=0\Rightarrow\,y{t}^{2}=2kt$$$$\Rightarrow\,y=\dfrac{2kt}{{t}^{2}}=\dfrac{2k}{t}$$$$\therefore\,$$co-ordinates of $$Q$$ are $$\left(0,\dfrac{2k}{t}\right)$$At $$x-$$axis we have $$y=0$$ at $$R$$$$\therefore\,x+0=2kt$$$$\Rightarrow\,x=2kt$$Hence coordinates of $$R$$ is $$\left(2kt,0\right)$$Midpoint of $$QR$$ is $$\left(\dfrac{0+2kt}{2},\dfrac{\dfrac{2k}{t}+0}{2}\right)=\left(kt,\dfrac{k}{t}\right)$$$$\therefore\,x=kt,\,y=\dfrac{k}{t}$$$$\Rightarrow\,xy=kt\times\dfrac{k}{t}={k}^{2}$$Hence the locus of the circumcentre of the triangle $$CQR$$ is $$xy={k}^{2}$$Maths

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