If the focus is (α,β) & the directrix is ax+by+c=0 then the equation of conic whose eccentricity =e is given by (x−α)2+(y−β)2=e2(ax+by+c)2a2+b2. If e=1 then conic is called parabola, for e<1 (conic is an ellipse) and for e>1, conic is a hyperbola.
Now consider the conic
169{(x−1)2+(y−3)2}=(5x−12y+17)2 ......(∗)
On the basis of above information answer the following question:
The equation of axis of the conic (∗) is
A
12x+5y+20=0
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B
12x−5y+17=0
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C
12x+5y+17=0
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D
12x+5y−27=0
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Solution
The correct option is D12x+5y−27=0 A conic is the locus of a point'P' which moves in such a way that its distances from a fixed point'S' always bears a constant ratio to its distances from a fIxed straight line. The fixed point 'S' is called focus. The fixed straight line is called directrix' & the constant ratio' is known as eccentricity denoted by e. ∴e=PS/PM Now 169{(x−1)2+(y−3)2}=(5x−12y+17)2 ⇒{(x−1)2+(y−3)2}=(5x−12y+17)252+(−12)2=(5x−12y+1713)2 ∴(x−1)2+(y−3)2=e2(5x−12y+1713)2 where e=1 Any line passing through focus (1,3) and perpendicular to 5x−12y+17=0 is the axis of conic (*). Now any line ⊥er to 5x−12y+17=0 is given by 12x+5y+λ=0 but it passes through focus ∴λ=−12(1)−5(3)=−27 ∴ equation of axis is 12x+5y−27=0