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How to prove the angle sum property of a triangle
Draw a line parallel to side bc of the triangle that passes through the vertex a.
Label the line pq. Construct this line parallel to the bottom of the triangle.
Write the equation angle pab + angle bac + angle caq = 180 degrees.
Remember, all of the angles that comprise a straight line must be equal to 180 . Because angle pab, angle bac, and angle caq combine together to make line pq, their angles must sum to 180 .