Hyperbola + Locus = Deadly Combination!

Geometry Level 2

A variable straight line of slope 4 4 intersects the hyperbola x y = 1 xy=1 at two points. The locus of the point which divides the segment between these points in ratio 1 : 2 1:2 can be written as a x 2 + 2 h x y + b y 2 + 2 g x + 2 f y = c ax^2+2hxy+by^2+2gx+2fy=c where a , h , b , g , f , c 0 a,h,b,g,f,c\geq 0 and gcd ( a , b , c , f , g , h ) = 1 \gcd(a,b,c,f,g,h)=1 .

Find a + b + c + f + g + h a+b+c+f+g+h .


Source: JEE advanced 1997.


The answer is 24.

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1 solution

Rushikesh Jogdand
Apr 16, 2016

Relevant wiki: Locus

Any point on the hyperbola x y = 1 xy=1 can be written as ( t , 1 t ) (t,\frac{1}{t}) .

Let for a particular line the intersection points be ( t 1 , 1 t 1 ) \left(t_1,\frac{1}{t_1}\right) and ( t 2 , 1 t 2 ) \left(t_2,\frac{1}{t_2}\right) 1 t 2 1 t 1 t 2 t 1 = 4 t 1 t 2 = 1 4 \therefore \frac{\frac{1}{t_2}-\frac{1}{t_1}}{t_2-t_1}=4 \implies t_1t_2=-\frac{1}{4} Required point P ( 2 t 1 + t 2 3 , 2 t 1 + 1 t 2 3 ) \text{P}\equiv\left(\frac{2t_1+t_2}{3},\frac{\frac{2}{t_1}+\frac{1}{t_2}}{3}\right)
Substituting t 2 = 1 4 t 1 t_2=-\frac{1}{4t_1}
P ( 2 t 1 1 4 t 1 3 , 2 t 1 4 t 1 3 ) ( x , y ) \text{P} \equiv \left(\frac{2t_1-\frac{1}{4t_1}}{3},\frac{\frac{2}{t_1}-4t_1}{3}\right)\equiv(x,y) 3 x = 2 t 1 1 4 t 1 ; 3 y = 4 t 1 + 2 t 1 3x=2t_1-\frac{1}{4t_1} \quad ; \quad 3y=-4t_1+\frac{2}{t_1} Eliminating t 1 t_1 we get,
2 x + y = 1 2 t 1 . . . [ 1 ] 2x+y=\frac{1}{2t_1}\quad...[1] Eliminating 1 t 1 \frac{1}{t_1} we get
8 x + y = 4 t 1 . . . [ 2 ] 8x+y=4t_1\quad...[2] [ 1 ] × [ 2 ] ( 8 x + y ) ( 2 x + y ) = 2 [1]\times[2]\implies (8x+y)(2x+y)=2 16 x 2 + y 2 + 10 x y = 2 \implies 16x^2+y^2+10xy=2 Thus, answer is 24 \boxed{24}


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