An interactive animation demonstrating the proof of the fact that the sum of internal angles of any triangle is 180º. Click the numbered buttons to observe the steps required for the proof.

http://www.absorblearning.com/media/item.action?quick=eg

For more interactive resources from Absorb Mathematics, visit
http://www.absorblearning.com/media/search.action#search

A top-notch collection of activities to reinforce some geometry theorems:
* angles on a straight line
* adjacent angles
* complementary angles
* supplementary angles
* vertically opposite angles
Three thumbs up!

Click on the magnifying glass to fill the screen (or interactive whiteboard)

http://www.mathbuddyonline.com/lessons/math/geometry/angles/pairsofangle...

This is a neat look-see proof that the sum of the exterior angles of any polygon is 360 degrees.
http://www.ies.co.jp/math/java/geo/gaikaku/gaikaku.html

Move the triangle by dragging the red point. What did you find?
http://www.ies.co.jp/math/java/geo/san180/san180.html

You are given a circle of radius 1 unit and two circles of radius a and b which touch each other and also touch the unit circle. Prove that you can always draw a 'flower' with six petals with the unit circle in the middle, and six circles around it having radii
a, b, b/a, 1/a, 1/b and a/b
such that each outer circle touches the unit circle and the two circles on either side of it.
http://nrich.maths.org/public/viewer.php?obj_id=2153

A wonderful interactive implementation of tesselations from the NRich website.
http://nrich.maths.org/public/viewer.php?obj_id=6069

A TI-Nspire activity for investigating opposite angles and angles formed with two lines are cut by a transversal.
http://www.tigeometry.com/activities/cabrijr/239/