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  Course: High school geometry>Unit 4
  
  Lesson 6: Proving relationships using similarity
  
  Proof: Parallel lines divide triangle sides proportionally
  
  Prove that a line parallel to one triangle side divides other sides proportionately. Created by Sal Khan.
  
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  • At , why did Sal put angle 2 and angle 4 in double arcs instead of single arcs?
  • It’s just a way of expressing the angles are congruent. If the single arcs are already being used, the double arcs are the thing you would use next - but it dosen’t really matter
  • Around in the video, I don't understand how Sal gets the EC and DB in the fractions.
  • He starts from AE*BD=AD*EC. As he states, he divides both sides by EC first to give AE*DB/EC=AD*EC/EC, they cancel on the right, then he does the same thing by dividing by DB which cancels on the left to get the fractions.
  • I just wanted to know why he continued the proof after saying that both triangles were similar since if they are similar then doesn't that mean that all 3 angles of 1 triangle each have a corresponding and equivalent second angle and the same thing with the sides except that they are propionate and not equivalent?
  • If you state that N lines are of equal length, by the number of slashes you put in them. And for angles you add extra rings/slashes. (hypothetically) Is there a generally accepted way to visually state that line 'AB' of triangle ABC is proportionate to line 'XY' of triangle XYZ? It would really help me with writing notes.
  • I don't know of any common way of noting it on a diagram, but if AB is proportional to XY, you could use a symbol used for proportionality: "∝". AB is proportional to XY would look like: AB ∝ XY.
  • Isn't this also known as the basic proportionality theorem ?
  • What does "leveraging" a triangle mean?
  (1 vote)
  • To leverage the diagram just means to make use of it. Studying the diagram may be helpful to solving the problem.
  (1 vote)
  • is this the thales theory?
  (1 vote)
  • No, Thales theorem says that if you inscribe a triangle in a circle, and one if its sides is a diameter of the circle, then the angle opposite that side is right.
  (1 vote)
  • When you insert the numbers given into the proof..One side is 1/3 while the other is 1/4 ?
  (1 vote)
  • The numbers shown in the videos are representing the angles, not the side measurements.
  (1 vote)
  • I cant make heads or toes what he means.