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Course: Geometry (all content)>Unit 12

Lesson 2: Introduction to triangle similarity

Determining similar triangles

Multiple examples where Sal analyzes the similarity of triangles. Created by Sal Khan.

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• At would that be considered an "improper" fraction because there are square roots in the denominators?
• No, because he assumes that you know to multiply the square roots by themselves to cancel out. An example : the square root of 3 times the square root of 3 is 3. That's how he cancels out the square root in the denominator.
• Why is it important to write the names of similar triangles in a certain order? Isn't triangle ABC the same as triangle CAB? Would similarity be affected?
• I think it's a good habit to arrange your triangle names. So if I told you that △ABC ≅ △XYZ, you don't have to look at a diagram to know that A and X are corresponding vertices in that congruence since they are both the first letters. If you do that, it's much easier to double check your CPCTC conclusions and that ASA or whatever was valid based on the evidence you gave.
• So we can't flip the figure... right? For example, the last question. It seems so tempting to be similar but it's not, unless we flip it. Is it allowed?!
• even if u flipped it the angle is not the included angle (btwn the 2 sides) they are not similar
• What would that squiggly line be called? ∆ ABC ~ ∆XYZ (~) <---- Would it be called similar?
(1 vote)
• That does mean similar, which I thought that Sal Khan had mentioned it in the video, when he says "∆ABC ~ ∆XYZ is called similar."
• is AA rule a subset of AAA rule?
• They are essentially the same thing. This is because if you know 2 of the angles in a triangle, then you also can determine the third (because they must add up to 180˚).
• if it is proved by sss similarity postulate that
triangle ABC is similar to triangle EDF
then does it mean that angle A is equal to angle E?
• Yes exactly. Make sure you match up the angles correctly though.
• if it is proved by sss similarity postulate that
triangle ABC is similar to triangle EDF
then does it mean that angle A is equal to angle E?
• Yes, if both are SSS congruent triangles, and when put onto one another through angle preserving transformations, point A is on Point E, then yes. (aka If I can put point A on E and have the other points match up, then yes._
• The angle PQR and DEF for the correspondence PQR and EDF.The perimeter of angle
ABC is 15 and perimeter of angle PQR is 27.if BC=7and QR=9,find PR and AC.
• 🐶🌎✨⚡️🔥🍉🌭🍕🍟🌯🍦🍰🍭🍿🏹🎲🎮🌋💵⚙💎?
aka if another triangle is double of another triangle does that mean their similar 100%?
(1 vote)
• Rueben, your argument is confusing because he uses the similar language, not congruent and you support his conclusion.
If there is a scale factor of 2 (that is ALL sides are doubled) as appears to be the original question, then yes the two triangles are similar just as you noted - I just do not know what 100% means, because either two triangles are similar (I guess that would be 100%) or they are not (I guess 0%) - there is no such thing as 50% similar, so the 100% does not add to the question