Existential threat

Geometry Level 3

It is given that a circle touches the line x + y 2 = 0 x+y-2=0 at ( 1 , 1 ) (1,1) and cuts the circle x 2 + y 2 + 4 x + 5 y 6 = 0 x^2+y^2+4x+5y-6=0 at P P and Q Q .

Can P Q PQ be perpendicular to the line x + y 2 = 0 x+y-2=0 ?

No Yes

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1 solution

Hosam Hajjir
May 13, 2018

If PQ is perpendicular to the given line, then the line segment joining the centers of the two circles must be parallel to the the given line.

Since the first circle is tangent to x + y 2 = 0 x + y - 2 = 0 at ( 1 , 1 ) (1,1) , then its center lies on the line y = x, thus its coordinates are ( t , t ) , t 1 (t, t) , t \ne 1 . The center of the second circle is ( 2 , 5 2 ) (-2, -\frac{5}{2} ) . The line joining them has a slope of m = t + 5 2 t + 2 m = \dfrac{ t + \frac{5}{2} }{ t + 2 } . Setting m = 1 m = -1 and solving for t t , we obtain, ( t + 2 ) = t + 5 2 - (t+2) = t + \frac{5}{2} , resulting in t = 9 4 t = -\frac{9}{4} . So our first circle must be chosen as ( x + 9 4 ) 2 + ( y + 9 4 ) 2 = 25 8 (x + \frac{9}{4} )^2 + (y + \frac{9}{4} )^2 = \dfrac{25}{8} , which when expanded, results in x 2 + 9 2 x + 81 16 + y 2 + 9 2 y + 81 16 = 25 8 x^2 + \frac{9}{2} x + \frac{81}{16} + y^2 + \frac{9}{2} y + \frac{81}{16} = \frac{25}{8}

Subtracting the equation of this circle from the equation of the second circle yields, 1 2 x + 1 2 y = 13 -\frac{1}{2} x + \frac{1}{2} y = 13 . This line is the line on which P and Q lie, it is easy to check that this line is perpendicular to the given line x + y 2 = 0 x + y - 2 = 0 . Hence, the answer is Y e s \boxed{Yes} .

Firstly confusion, how did you write solution if the answer you give is 'yes' while the correct answer is 'no'?

Akshat Sharda - 3 years ago

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At first, I thought the answer is No., but when I wrote the solution in detail, I found out the answer is actually Yes.

Hosam Hajjir - 3 years ago

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The answer actually comes out to be no. Why? Well, circle 1 comes out to be x 2 + y 2 + 9 2 x + 9 2 y 11 = 0 x^2+y^2+\frac{9}{2}x+\frac{9}{2}y-11=0 and after careful observation one can see that this circle and circle 2 are non intersecting.

I found this using the family of circle having x + y 2 = 0 x+y-2=0 tangent at ( 1 , 1 ) (1,1) which is ( x 1 ) 2 + ( y 1 ) 2 + λ ( x + y 2 ) = 0 (x-1)^2+(y-1)^2+\lambda (x+y-2)=0 . Now subtracting circle 2 from this, we get x ( λ 6 ) + y ( λ 7 ) + 8 2 λ = 0 x(\lambda -6)+y(\lambda -7)+8-2\lambda=0 . For this to be perpendicular to x + y 2 = 0 x+y-2=0 , λ \lambda equals 13 2 \frac{13}{2} . Now, after observation, the distance between the centre's of the two circles comes out to be less r 1 r 2 |r_1 -r_2| . Hence, they come out to be non intersecting.

Akshat Sharda - 3 years ago

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@Akshat Sharda Oh. I see. You're absolutely right.

Hosam Hajjir - 3 years ago

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