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# CA Geometry: Similar triangles 1

10-14, similar triangles. Created by Sal Khan.

## Want to join the conversation?

• at about "" what is the definition of similar triangles
• it means that the triangles are the same thing, but they might be bigger or smaller.
(1 vote)
• weren't all the sides on the paralellogram on the same?
• There are four paralellograms: Rhombus, Rhomboid, Rectangle, and a Square. The square is the only paralellogram must have four equal sides. While a Rhombus can have 4 equal lengths, for this problem, it is not a given that this is the case. We are looking for proof only that triangles inside the paralellogram are congruent. The answer to your question is the angles could be the same, either way, you should be able to get the right answer even if they were not.
• I don't get problem 13 at
Can anyone help me?
(1 vote)
• For the 2 triangles to be similar the corresponding angles all have to have congruent measurements. For obvious reasons, angle DBE is the same on both tri. For the other 2 corresponding angles to be proven to be congruent, the 2 sides AC and DE must be parallel to eachother. This is true because the transversal of line AB would make the 2 corresponding angles of the triangle congruent ecause of the corresponding angles of the transversal are these angles. This is true for the other transversal as well.
• In problem #10, minute , wouldn't the triangles be called RTP or PTR or just T for the one in purple, and the other to be called APT or TPA or just P. I thought that the major/main angle of the triangle needed to be in-between the other two letters when naming it?
• I'm not aware of defining a triangle based solely on the reference to the major angle.
• Do the arrows on line segments AC and BE at represent parallel line segments?
• Yes, the arrows on line segments AC and BE do denote parallel line segments.
• Sal could have also used the SSS, couldn't he?
• You could do that, but first you would have to prove that the third side of the left triangle (RP) is equal to the third side of the right triangle (TA), and we aren't given that.

However, there is a rule when it comes to isosceles trapezoids which states that the diagonals on an isosceles trapezoids are always congruent. So if you knew that, then you would be able to state that, by this rule, RP must equal TA, and therefore you could use SSS.

However, if you already knew that RP equaled TA, then you wouldn't even need to prove that the triangles are congruent. Because "RP = TA" is the solution to the problem. So if you knew that, you would already be done.

I hope this helps.
• how do i use proofs like how do i write them and solve them