The sum of the angles should equal 180
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Exterior angle.
the exterior angle theorem
An exterior angle of a triangle is equal in measure to the sum of the other two interior angles.
Theorem 6-1-2; Polygon Exterior Angle Sum Theorem:The sum of the exterior angle measures, one angle at each vertex, of a convex polygon is 360 degrees.
When any side of triangle is extended outwards then exterior angle is formed. Sum of this exterior angle and adjacent interior angle = 180o. If exterior angle = 180o(straight angle) then interior adjacent angle is 0o which is not possible. So exterior angle can't be straight angle.