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# Worked example: Classifying triangles

Work through sample problems categorizing triangles as scalene, isosceles, equilateral, acute, right, or obtuse. Created by Sal Khan.

## Want to join the conversation?

- If I have two angles that make no sense like 30, 130 and the third

one is missing how do I find the answer?(7 votes)- All the angles of a triangle add up to 180 degrees, so if you have two of the measures, you add up the two measures given and then subtract it from 180 to get the third measure. If you want to test if a triangle adds up to 180 degrees, you can cut out a triangle out of paper and then tear of the three corners. Then, on another piece of paper, you draw a straight line because a straight line is 180 degrees. You put all the corners of your triangle on the line and it should fit.(24 votes)

- I'm going to 4th grade this year and I want to improve my reading. And also, I'm looking forward to getting mostly 4’s and 3's on my report card (We grade by 1,2,3,4 with 1 being the worst and 4 being the best.) Any Tips? Thank you very much.(6 votes)
- Practice, practice, practice. Make a study schedule and stick to it. Go over notes you get in class and if you have a hard time understanding something you read or something in class, read it again or ask the teacher to explain it. Ask relevant questions, that always helps. DO NOT fall asleep in class.

To improve in reading, try reading books that challenge you but don't overwhelm you. Experiment a bit to find what books would interest you specifically. If your up to it, try going for books with different styles. By style I mean how its spoken and written. Examples are British vs. American (slight differences in dialect). Or Shakespearean vs. slang (HUGE DIFFERENCE). This helps broaden vocabulary and will definitely help you understand plot and setting better.

Hope this helped. Good luck in fourth grade!!(16 votes)

- Can a equilateral triangle be a scalene triangle?(3 votes)
- A scalene triangle means
*all sides have a different length*. An equilateral triangle means all sides are the same length.(9 votes)

- Obtuse what is it i don't know?(4 votes)
- obtuse angles are angles which measures above 90 degrees and below 180 degrees(7 votes)

- at0:39I didn't hear what he was saying(3 votes)
- If you ever have trouble understanding what Sal is saying, you can use the transcripts below the videos.

Around0:39he said: "...we can assume that these are drawn to scale, so we can base it on our visual judgement. So I would say that this is obtuse..."(6 votes)

- i need help on something ules than tis(2 votes)
- "Boi" is not a word in the English dictionary(5 votes)

- Why do we have to classify shapes?(2 votes)
- It makes them a lot easier to talk about if they have names and labels that we can agree on.(5 votes)

- in the part were u made us guess the triangles you should have switched the order so we could actully guess but this is well it helped thank u(4 votes)
- this is very good stuff(4 votes)
- you are pretty we Love you no matter what you pretty like the stars and we love God and we are 4th grade we are pretty we can read we can write and do anything.(3 votes)
- correct that is true we love that(2 votes)

## Video transcript

Categorize the following
triangles according to whether or not they
are obtuse triangles. So an obtuse triangle
is a triangle that has an obtuse
angle in it, or an angle that is larger than 90 degrees. So it's pretty clear
that this one does not have any obtuse angles. This is a 90 degree
angle, and these are going to have to be
less than 90 degrees. So this is not obtuse. This one right over here,
just by looking at it, you see that all of them
are less than 90 degrees, so I'll put this not obtuse. Now, these two are interesting. Angle JKL looks like it
is larger than 90 degrees, and we can assume that
these are drawn to scale, so we can base it on
our visual judgment. So I would say that
this is obtuse, and angle BAC also looks like
it is larger than 90 degrees. So that's an obtuse
angle, so I would throw that in the
obtuse bucket as well. So we had two not
obtuse, two obtuse. Check our answer. Let's do a couple more of these. Which of the following
are correct descriptions of triangle PIG? Or I guess triangle pig? Select all that apply. PIG is equilateral. Well, that's not true. To be equilateral, all the sides
have to be the same length. And we see here two
sides are seven, but one side has length 4. So that's not true. Angle PIG has two equal angles. Well, we see that
right over here, these two angles
that are 74 degrees. So that's true. Triangle PIG, I guess,
has an obtuse angle. Well, an obtuse angle is one
that's larger than 90 degrees. None of these are
larger than 90. That's not true. Triangle PIG has
three acute angles. Well, that is true. All of these angles are
less than 90 degrees. Triangle PIG has a right angle. No, none of these angles
are exactly 90 degrees, so it does not
have a right angle. So let's check our answer. Let's do one more of this. This is actually kind of fun. Categorize the following
triangles according to whether or not they
are equilateral. So to be equilateral,
all of the sides have to have the same length. So this is equilateral. This one, the sides are
definitely not the same length. In fact, not even two of the
sides are the same length. That's really a
scalene triangle. Here we have two sides
that are the same length, but the third is different. This would be an
isosceles triangle, but it's not equilateral. And here they're
all the same length, so we have an
equilateral triangle. We got it right.