If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Worked example: Triangle angles (intersecting lines)

Worked examples finding angles in triangles formed by intersecting lines. Created by Sal Khan.

## Want to join the conversation?

• Can't you just assume that the intersecting lines form a 90° right angle because they are perpendicular?
• No, because the lines aren't actually perpendicular.

You're right that if they were perpendicular, if that was given, then yes, they would form four 90 degree angles by the definition of perpendicular lines. However, as the lines aren't given to be perpendicular (and over the course of solving the problem, we find that they aren't) we can't just assume.

It's VERY important in Geometry to NEVER assume that lines are perpendicular (or parallel, or anything like that) just because they appear that way - (I learned that the hard way last year LOL).

Hope this helps!
• Couldn't Sal just have subtracted 121 from 180 to find 59 degrees on the left side and say that this pair of supplementary angles are congruent to the ones found at angle x? That could save so much time during a test!
• That was kind of my reasoning, Sathvik. Due to the 2 given 29° angles of the traversal, I reasoned the 2 lines are parallel, which would make angle "x" supplementary to the 121° angle.

Supplementary means x + 121° = 180°

Much easier.
• Can someone please explain, At , why he subtracted 121 from 29? I don't get it, is there a rule that I've missed out? Doesn't make any sense to me. Thank you .
• So, you know that all the angles in any triangle add up to 180 degrees (I assume). To find the missing angle, you need to subtract the given angles from 180, and you will find the missing angle. The reason that Sal chose to subtract from 121 is because instead of subtracting 121 from 180 to find the inner angle (because they are supplementary, add up to 180 degrees) and then subtracting whatever he got from 180, you know that the answer will be 121. Then, after subtracting one inner angle from 180, which is essentially what he did, he subtracted the other inner angle (29) from 121. I hope this helps!
• When will we use this in real life (i want to know)
• Right now you're learning the basics. Later you'll apply it to different and more advanced levels of math that have more relevant real-life applications.
• Can anyone tell me what is the meaning of supplementary and complementary angles?
• Supplementary angles add up to 180 degrees (straight lines also have a measure of 180 degrees) and complementary angles add up to 90 degrees. If you need more confirmation, you can go to a Khan Academy video about complementary and supplementary angles.
• Hi! I just did 180-121 and got 59 when he told me to try it on my own. Is that an ok way to do it?
• That's a great way to do it!
(1 vote)
• what is a complementary angle