Properties of cicles

This topic has expert replies
User avatar
Master | Next Rank: 500 Posts
Posts: 184
Joined: Sun Aug 19, 2012 10:04 pm
Thanked: 10 times
Followed by:2 members

Properties of cicles

by Mission2012 » Sun Sep 01, 2013 7:40 pm
Is it true -

Only way two perpendicular lines can be tangent to the same circle if the arc they form represent exactly one quarter of a circle.
If you find my post useful -> please click on "Thanks"
Source: — Problem Solving |

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Sun Sep 01, 2013 10:32 pm
Hi Mission2012,

I'm going to "tweak" your sentence a bit, but I can tell you that the idea that you're talking about IS true.

When two lines that are tangent to a circle and perpendicular to one another, then the arc of the circle that exists between them will be 1/4 of the circumference.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image

Senior | Next Rank: 100 Posts
Posts: 81
Joined: Tue Jun 11, 2013 10:24 pm
Thanked: 7 times
Followed by:1 members

by sanjoy18 » Sun Sep 01, 2013 10:39 pm
actually two perpendicular tangents form an angle of 90 at center of circle
therefore length of minor arc 90/360=1/4
hence proved

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 768
Joined: Wed Dec 28, 2011 4:18 pm
Location: Berkeley, CA
Thanked: 387 times
Followed by:140 members

by Mike@Magoosh » Mon Sep 02, 2013 10:33 am
Mission2012 wrote:Is it true -

Only way two perpendicular lines can be tangent to the same circle if the arc they form represent exactly one quarter of a circle.
Dear Mission2012,
I happy to add my two cents. :-)

Yes, this statement is true. For example, suppose the circle has a center O, the two tangents are tangent at points A & C, and the tangents intersect at point B outside of the circle. We already know angles A & C, where the radius meets a tangent, are 90 degree angles. If B is also a right angle, that's a quadrilateral with three right angles --- since the sum of the angles must be 360 degrees, then if three of the angles are right angles, the fourth must be a right angle also. In fact, because all the lengths are equal, quadrilateral OABC has to be a square.

This is a rare case in which a relatively simple specification guarantees the quadrilateral formed is a square. Normally, guaranteeing something is a square is not so easy. See:
https://magoosh.com/gmat/2012/gmat-geome ... -a-square/

Does all this make sense?
Mike :-)
Magoosh GMAT Instructor
https://gmat.magoosh.com/