a ppt is here
1. a sequence,
- starting with an isosceles right angled triangle (maybe with reference to this being half a square)
- a right angled triangle (being half a rectangle)
- an isosceles triangle (using two of (ii))
- an acute angled triangle (also using two of (ii) but different triangles)
- an obtuse angled triangle (made up with two right angled triangles)
(2) folding a triangle and involving reflection
(3) the usual proof
create a line parallel to the base, through the apex
then involve angles on parallel lines (alternate angles) which can be justified by rotation
this is maybe more evident on a tessellation of triangles
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