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The three interior angles of any planar triangle always add to 180°, so knowing two of them fixes the third as 180° minus their sum.
Angle sum
A + B + C = 180° ⇒ C = 180° − A − B
Drawing a line through one vertex parallel to the opposite side creates alternate angles equal to the other two, and together they form a straight line.
No. On a sphere the angles of a triangle sum to more than 180°, and the excess is proportional to the triangle's area.