Sum of Triangle's angles = 180 degrees

 
F + C = 180
A + D = 180
B + E = 180
So   A + B + C + D + E + F = 540
Take out the triangle section and the three exterior angles can slide
to meet in the center.
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Now you can see D + E + F = 360
This with  A + B + C + D + E + F = 540
means A + B + C + 360 = 540
So A + B + C = 180
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Hop's Gallery - Please check out my artwork.
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Hop David Coloring Books
tessellation coloring book
A Hop David coloring book
of Escher like tessellations

Geoscapes coloring book
A Hop David coloring book
of various geometrical landscapes.

To be released in February of 2009.
A lot of pages are devoted to space filling polyhedra with an emphasis on octahedra alternating with tetrahedra.
There's also some polyhedra studies, logarithmic spirals, fractals (only the first few iterations or they'd be impossible to color!) and some miscellaneous.
This book includes a CD. I think these coloring books will be a good tool for teachers in helping get kids interested in math.