• October 9th 2008, 04:47 AM
xwrathbringerx
Two circles touch internally at a point A and the smaller of the two circles passes through O, the centre of the larger circle. AB is any chord of the larger circle, cutting the smaller circle S. The tangents to the larger circle at A and B meet at a point T.

http://www.mathhelpforum.com/math-he...55-circles.bmp

Prove:

i) AB is bisected at S.

ii) O, S and T are collinear.

• October 9th 2008, 07:39 AM
Soroban
Hello, xwrathbringerx!

Quote:

Two circles touch internally at a point $A$
and the smaller of the two circles passes through $O$, the centre of the larger circle.
$AB$ is any chord of the larger circle, cutting the smaller circle $S.$
The tangents to the larger circle at $A$ and $B$ meet at a point $T.$

http://www.mathhelpforum.com/math-he...55-circles.bmp

Prove:

a) $AB$ is bisected at $S.$

b) $O, S\text{ and }T$ are collinear.

Draw $OT.$
Draw radii $OA = OB = r$

$TA = TB$
Tangents to a circle from an extrenal point are equal.

Points $O$ and $T$ are equidistant from points $A$ and $B.$
. . Hence, $OT$ is the perpendicular bisector of $AB.$

$OA$ is a diameter of the small circle.
$\angle OSA$ is inscribed in a semicircle: . $\angle OSA = 90^o$

Hence, $S$ lies on $OT.$

And the two proofs follow . . .

• October 9th 2008, 07:36 PM
xwrathbringerx
Can I simply say OA is a diameter of the small circle?