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### Course: Basic geometry and measurement > Unit 4

Lesson 2: Classifying triangles# Types of triangles review

Review ways to classify triangles based on their sides lengths and angles. Practice classifying triangles.

## Classifying triangles by their angles

### Acute triangles

An ${\text{acute}}$ triangle has ${\text{3 angles that}}$ ${\text{each measure less than}{90}^{\circ}}$ . Below are examples of ${\text{acute triangles}}$ .

### Right triangles

A ${\text{right}}$ triangle has ${\text{1 angle that measures}{90}^{\circ}}$ and $2$ acute angles. Below are examples of ${\text{right triangles}}$ .

### Obtuse triangles

An ${\text{obtuse}}$ triangle has ${\text{one angle that measures}}$ ${\text{more than}{90}^{\circ}}$ and $2$ acute angles. Below are examples of ${\text{obtuse triangles}}$ .

*Want to learn more about classifying triangles? Check out this video.*

*Want to try more problems like this? Check out this exercise*

## Classifying triangles by their side lengths

### Equilateral triangles

An ${\text{equilateral}}$ triangle has ${\text{three equal sides}}$ . Below are examples of ${\text{equilateral}}$ triangles.

### Isosceles triangles

An ${\text{isosceles}}$ triangle has ${\underset{\u2015}{\text{at least}}}$ ${\text{two equal sides}}$ . Below are examples of ${\text{isosceles}}$ triangles.

### Scalene triangles

A ${\text{scalene}}$ triangle has ${\text{no equal sides}}$ . Below are examples of ${\text{scalene}}$ triangles.

*Want to try more problems like this? Check out this exercise.*

## Want to join the conversation?

- Why can't I go back to selecting? I got a wrong and I can't go back(80 votes)
- It just won't let you.(26 votes)

- why is the isosceles triangle and the equilateral both classified as each other(49 votes)
- because an equilateral triangle is all equill sides and an isosceles triangle is two or more equill sides(20 votes)

- Is their any more triangle practices?(24 votes)
- No. You will have to do them over if you want extra practice.(13 votes)

- Why are triangles used in math?(17 votes)
- it is because it uses geometry(10 votes)

- I don’t understand why an equilateral triangle is constantly being defined as and Isosceles triangle.(9 votes)
- An equilateral triangle has 3 equal length sides. An isosceles triangle has at least 2 sides of the same length. Thus all equilateral triangles are also isosceles.(31 votes)

- if all of the angels are acute but lenghths are different what is that?(7 votes)
- i got my one right but it said i got it worng(13 votes)
- I... I think that means you got it wrong, but you don't understand. You might want to check your answer or ask someone else to explain it to you.(5 votes)

- um whenever i double tap a question it ameatiatily says its wrong(11 votes)
- dont double tap just tap and check(7 votes)

- Why do all sides measurements in a triangle have to equal 180(4 votes)
- Nice question! I think you meant to ask why all the angle measurements in a triangle add to 180 degrees. The side measurements can add up to anything.

There is a geometry proof of the fact that the angles in a triangle must add to 180 degrees.

Draw a triangle with vertices at points A, B, and C, and draw a line through vertex C that is parallel to side AB. This diagram forms three non-overlapping angles at vertex C. Two of the angles are just outside the triangle and one of the angles is angle C inside the triangle.

These three non-overlapping angles formed at vertex C clearly add to 180 degrees (because together, they form a semicircle).

Because alternate interior angles are congruent when parallel lines are cut by another line (transversal), angle A in the triangle is congruent to one of the angles at C outside the triangle, and angle B in the triangle is congruent to the other angle at C outside the triangle.

It follows from these last two statements that angles A, B, and C in the triangle add to 180 degrees.

Have a blessed, wonderful day!(13 votes)

- it said that a acute was like all the sides lower then 90* but the last one was over 90*(8 votes)
- then it was obtuse(5 votes)