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The measure of an exterior angle is equal to the sum of the remote interior angles.

Proof:

Givens are angles a, b, c, 1, 2, 3.

  \
   \
    \1______________________
     \a             b / 2
      \              /  
       \            /
        \          /
         \        /
          \      /
           \    /
            \c /
             \/
           3 /
            /
           /

  1. a+b+c = 180° (by sum of angles of triangle)
  2. b+c = 180-a (by subtraction)
  3. a+1 = 180° (by linear pair postulate)
  4. 1 = 180-a (subtraction)
  5. 1 = b+c (by substitution).

Q.E.D.

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