angles in a triangle (zooming)
Explore angles in a triangle by zooming in and out. 
What do you notice about the 3 angles when you keep zooming out until the triangle becomes very small?
What do you notice about the 3 angles when you keep zooming out until the triangle becomes very small?Here is a quick demo.
The effect of zooming is the same as moving the lines towards the centre without rotating (translation). The angles between each pair of lines will not change. If the three lines eventually meet at a point, the angles inside the triangles do not disappear. They can still be identified as angles between each pair of lines. By considering one pair vertically opposite angles, we can deduce the sum of those 3 angles.