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.

 

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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