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In Euclidean geometry, the sum of the angles Î± + Î² + Î³ is equal to two right angles (180Â° or Ï radians). This allows determination of the third angle of any triangle as soon as two angles are known. should IMHO be: In Euclidean geometry, the sum of the internal angles is a straight angle. This allows determining the amplitude of the third internal angle of any triangle once the amplitudes of the others are known. Similar confusions exist between segments and their lengths.
We know that a straight line always measures 180 degrees. So because your interior and exterior angles together make up a straight line, they are supplementary. You know how to find the measure of your interior angle, so your exterior angle is simply 180 - Interior Angle. For the triangle, 180 - 60 = 120. 120 degrees is the measure of the exterior angle.