Theorem 1.14: If a line is perpendicular to one of two parallel lines then it is perpendicular to the other.

Proof: If a line is perpendicular to one of two parallel lines then its slope is the negative reciprocal of the slope of the one parallel line. Since parallel lines have the same slope, its slope will then also be the negative reciprocal of the slope of the other parallel line, and so it will be perpendicular to the other parallel line.

next theorem (1.15)