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## Geometry (all content)

### Course: Geometry (all content)>Unit 12

Lesson 3: Solving similar triangles

# Solving similar triangles

Sal solves two problems where a missing side length is found by proving that triangles are similar and using this to find the measure. Created by Sal Khan.

## Want to join the conversation?

• Why do we need to do this? Will we be using this in our daily lives EVER?
• Similarity and proportional scaling is quite useful in architecture, civil engineering, and many other professions. You will need similarity if you grow up to build or design cool things.
• Can someone sum this concept up in a nutshell? I'm having trouble understanding this.
• There are 5 ways to prove congruent triangles. SSS, SAS, AAS, ASA, and HL for right triangles. To prove similar triangles, you can use SAS, SSS, and AA.
• What is cross multiplying? (at )
• Cross-multiplying is often used to solve proportions. As an example:
14/20 = x/100
Then multiply the numerator of the first fraction by the denominator of the second fraction:
1400 =
Then, multiply the denominator of the first fraction by the numerator of the second, and you will get:
1400 = 20x.
Solve by dividing both sides by 20. The answer is 70.
• what are alternate interiornangels
• Between two parallel lines, they are the angles on opposite sides of a transversal. Hope this helps!
• in the 2nd question of this video, using c&d(componendo&dividendo), can't we figure out DE directly?
• We could, but it would be a little confusing and complicated. For instance, instead of using CD/CE at , we could have made it something else that would give us the direct answer to DE. We could have put in DE + 4 instead of CE and continued solving. But it's safer to go the normal way.
• Is this notation for 2 and 2 fifths (2 2/5) common in the USA? I´m European and I can´t but read it as 2*(2/5).
• How do you show 2 2/5 in Europe, do you always add 2 + 2/5? We would always read this as two and two fifths, never two times two fifths.
• In geometry terms, do congruent figures have corresponding sides with a ratio of 1 to 2?