Circle and a parabola

the common chord of a circle with centre (-4,0) and a parabola y2=8x subtends an angle 90 at the vertex.Find the radius of the circle.

12 Answers

1
Prashant Shekhar ·

2√13

1
skygirl ·

one thing, the chord of contact will be symmetrical wrt x-axis..
(since both circle n parabola are symmetrical to x-axis..)

let radius be 'r'.

take the two extremities of chord of contact as A(rcosθ1, rsinθ1)
and B(rcosθ2, rsinθ2).

but since the line is symm with x-axis, θ2=θ1.
so the two points become A(rcosθ1, rsinθ1) and B(rcosθ1, -rsinθ1).

given that the ch of contact subtends a r8 angle at vertex i.e. (0,0)

so m1.m2 =-1.
from here we get sin2θ1/cos2θ1 = 1
=> tanθ= +- 1. => θ=Π/4, -Π/4.

These points also lie on the parabola.

so (rcos pi/4 , rsin pi/4) must satisfy parabola eqn.

then r= 4√2.

1
Prashant Shekhar ·

you should note that the centre of the circle is not origin but is at (-2,0)

1
Prashant Shekhar ·

1
skygirl ·

:(
yeah i missed that...

we have to take the points -2+rcosθ1,0+rsinθ1 .

shit.

1
Prashant Shekhar ·

Co-ordinates of the end points of the chord can be found by what you suggested.

The radius will simply then be the distance between either of these points to the centre of the circle.

33
Abhishek Priyam ·

Let the eqn of common chord be x=k .........(why?)

homogenize it with y2=4x

to get y2-4x2/k

Now 1-4/k=0...
so k=4

so chord is x=4...
It cuts parabola at (4,4)

radius of circle is distance b/w (-2,0) and (4,4) which is 2√13....

1
Prashant Shekhar ·

say the cordinates are (h,k) and (h,-k)

since product of slopes =-1
implies k=±h

so the points will be (h,h) and (h,-h)

since it lies on parabola
implies h=4

hence radius =√62+42

33
Abhishek Priyam ·

Nice method :)

1
skygirl ·

wow.. gr8 method :)

1
voldy ·

mughe yeh samaj me nahi araha ki aap logon ne kaise centre of the circle ko (-2, 0) liya ?

the one shown in the diagram is also possbile no?

mera doubt clear karo koi ? sorry if it's dumb.

here's the figure.
this is also possible no?

33
Abhishek Priyam ·

Hey I think the question was edited after sonls.... so the solns are not looking appropriate

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