Angle 1 and angle 4 add up to 180 degrees because they are on the same straight line split into two parts, and a straight line is 180 degrees. Angle 4 and angle 7 are corresponding angles, which means they occupy the same relative position at each intersection where a straight line crosses two others. If the two lines are parallel, the corresponding angles are equal. Supplementary angles mean that any two angles add up to 180 degrees. Since 4 and 7 are corresponding, 1 and 7 must be supplementary
