Home » Ask & Discuss » Mathematics
« Back to Discussion

Trignometry

Administrator

Joined:	19 Oct 2006
Post:	1558

18 Nov 2006 03:16:23 IST

0 People liked this

2141

Contest [swordfish #4]: Find equation of tangents' mid point

Engineering Entrance , JEE Main , JEE Advanced , Mathematics , Trigonometry

From a point P (x₁ , y₁ ), tangents PA and PB are drawn to the circle x² +y² +2gx +2fy +c = 0
If P' and Q' are mid points of PA and PB respectively, find the equation of P'Q'.

Comments (17)

malay

Hot goIITian

Joined: 18 Dec 2006

Posts: 146

31 Dec 2006 11:45:14 IST

Like 0 people liked this

this problem consists of four small steps:
1)finding the equation of AB
2)finding M(the intersection point of AB and OP)
3)finding the mid point of PM
4)finding the equation of P'Q'
every step is a short step.
in the first step, let A be (x,y) and C be centre of OP. AC=CP completes first step
infourth step, slope and a point are known.

answer comes out to be x(x1+g)+y(y1+f)=PA/2

prats7575

New kid on the Block

Joined: 31 Dec 2006

Posts: 17

31 Dec 2006 19:34:21 IST

Like 0 people liked this

The Ans is 2x-x1+g=0

prats7575

New kid on the Block

Joined: 31 Dec 2006

Posts: 17

31 Dec 2006 19:40:07 IST

Like 0 people liked this

If c9-g,-f) is centre of circle,

First take c(-g,-f)

Then the middle point of CP, which is P' [(x1-g)/2,(y1-f)/2]

now the equation of PQ is the x = (x of the point P')

So, the equation is x=(x1-g)/2.

So, 2x- x1+g=0

tejal_iitian

New kid on the Block

Joined: 29 Dec 2006

Posts: 6

6 Jan 2007 19:53:31 IST

Like 0 people liked this

D EQN OF P'Q' IS 2x2 -x1 +g=0

chandrasekhar

Forum Expert

Joined: 12 Nov 2006

Posts: 25

20 Jan 2007 19:55:46 IST

Like 0 people liked this

swordfish #4 result I'm afraid, no student has been able to answer this question with the correct technique.

kaushaldaga

New kid on the Block

Joined: 23 Jan 2007

Posts: 4

24 Jan 2007 14:38:51 IST

Like 1 people liked this

LINE JOINING P'Q' IS PARALLEL TO CHORD OF CONTACT

EQN OF CHORD OF CONTACT IS xx'+yy'+g(x+x')+f(y+y')+c=0

=) x(x'+g)+y(y'+f)+gx'+fy'+c=0

:.P'Q'= x(x'+g)+y(y'+f)+k=0

ankur gupta

Scorching goIITian

Joined: 10 Mar 2007

Posts: 257

10 Mar 2007 21:55:38 IST

Like 5 people liked this

LINE JOINING P'Q' IS PARALLEL TO CHORD OF CONTACT

equation of chord of contact is :-

xx'+yy'+g(x+x')+f(y+y')+c=0

x(x'+g)+y(y'+f)+gx'+fy'+c=0

so the slope of the P'Q' will be = -(x'+g)/(y'+f)

centre C=(-g,-f)

as the mid -point ((-g+x')/2,(-f+y')/2)of the line joining CP lies on the line P'Q'

thus the eqn of P'Q' will be by point-slope form

2x(x'+g)+2y(y'+f)+g^2+f^2-x'^2-y'^2=0

thats the answer

Subho Banerjee

Hot goIITian

Joined: 8 Mar 2007

Posts: 153

10 Mar 2007 23:54:25 IST

Like 0 people liked this

ankur is correct!!

neelesh sahay

Hot goIITian

Joined: 16 Feb 2007

Posts: 128

11 Mar 2007 01:21:38 IST

Like 0 people liked this

ankur is absolutely correct..

sandy kr

New kid on the Block

Joined: 5 Jun 2007

Posts: 2

5 Jun 2007 16:41:06 IST

Like 0 people liked this

from the question for pt. P (X₁Y₁)

sandy kr

New kid on the Block

Joined: 5 Jun 2007

Posts: 2

5 Jun 2007 16:49:57 IST

Like 0 people liked this

LINE JOINING P'Q' IS PARALLEL TO CHORD OF CONTACT

equation of chord of contact is :-

xx'+yy'+g(x+x')+f(y+y')+c=0

x(x'+g)+y(y'+f)+gx'+fy'+c=0

now we can find the

distance of chord of contact to point P. from similiarity of triangle we will find that the

distance of P'Q' from P will be half of

distance of chord of contact to point P. so distance from P'Q' to P can be known. now we know slope of P'Q' & distance between both parrallel lines

so slope is known & parallel distance is known so we can find the eqn. of P'Q' .

sravan qwerty

New kid on the Block

Joined: 24 Aug 2007

Posts: 24

27 Aug 2007 11:57:00 IST

Like 0 people liked this

x =2 u=78 plzz helpm-e m=90 p=78

varun kurtkoti

New kid on the Block

Joined: 25 Dec 2007

Posts: 2

25 Dec 2007 13:38:36 IST

Like 0 people liked this

it is same as the equation of chord AB......

Celestine Preetham

Hot goIITian

Joined: 24 Apr 2008

Posts: 163

30 Apr 2008 20:38:49 IST

Like 0 people liked this

THE ans is Ti - S1/2

abhishek952

New kid on the Block

Joined: 27 Nov 2008

Posts: 2

27 Nov 2008 08:57:00 IST

Like 0 people liked this

it can be find outr by solving for54

Achiever...2011

Hot goIITian

Joined: 30 Mar 2009

Posts: 104

6 Aug 2010 20:08:23 IST

Like 0 people liked this

kammadanam prabhakar

New kid on the Block

Joined: 18 May 2012

Posts: 1

18 May 2012 11:21:03 IST

Like 0 people liked this

Contest [swordfish #4]: Find equation of tangents' mid point

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team