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Head First 2D Geometry by Dawn Griffiths, Stray

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triangle corners add up to 180
º
The corner angles of a triangle
always add up to a straight line
The corner angles of a triangle always make a straight line,
no matter what kind of triangle it is. Angles on a straight
line always add up to 180º, so the corner angles of a
triangle must always add up to 180º, too.
This is true for
every triangle
you could possibly
draw!
67º
78º
35º
10º
105º
65º
110º
32º
38º
50º
75º
55º
Try it with any
shape of triangle—
fat, thin, big, small....
20 Chapter 1

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