[header] How to prove the angle sum property of a triangle [title] Draw a line parallel to side bc of the triangle that passes through the vertex a. [step] Label the line pq. Construct this line parallel to the bottom of the triangle. [title] Write the equation angle pab + angle bac + angle caq = 180 degrees. 
[step] 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 °.