In the figure, Segment AB is a diameter of the smaller of the two concentric circles. Segments AP and BQ are tangent to the smaller circle at A and B, respectively.

PROVE that Segments AB and PQ intersect at the center of the circle.

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- Jan 24th 2011, 12:43 AMKalodaProving?
In the figure, Segment AB is a diameter of the smaller of the two concentric circles. Segments AP and BQ are tangent to the smaller circle at A and B, respectively.

PROVE that Segments AB and PQ intersect at the center of the circle. - Jan 24th 2011, 01:39 AMemakarov
For clarity, I made the circles very non-concentric in the following picture.

http://lh5.ggpht.com/_SNa3egOo9rk/TT...concentric.png

Let O be the center of the circles, by definition. Then AP = BQ by the Pythagoras theorem. Therefore, $\displaystyle \triangle AOP= \triangle BOQ$. So, $\displaystyle \angle AOP=\angle BOQ$, which means that POQ is a straight line. - Jan 24th 2011, 04:41 PMbjhopper
I did this proof as follows

given two concentric circles,AB diameter of small circle, AP and BQ tangents to small circle @A and B

1 extend PA and BQ to C and D points on the large circle

2 TAngents @ A and B meet diameter AB @ 90 deg property of tangents

3 PAC and QBD are parallel two lines perpendicular to a third line are parallel

4 PAC and QBD are equal in lenght Each is a chord of large circle perpendicular to AB and equidistant to center of circles m the midpoint of AB

5 PCQD is a rectangle CQ=PD =AB perpendicular distance between parallel are equal

6 QP and CD are diagonals of the rectangle meeting AB @ m the middle of rectangle Property of rect diagonals

7 QM =MP CM=DM AM- MB Mis the midpoint of AB and the point bisecting the three lines QP, CD, AB

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