51. Prove that if the slopes of two lines have product -1, then the lines are perpendicular. Use parts (a)–(c) to write a coordinate proof. a. First, argue that neither line can be horizontal or vertical. b. Then, tell why the lines must intersect. (Hint: Use indirect reasoning.) c. Place the lines in the coordinate plane. Choose a point on /1 and find a related point on /2. Complete the proof.