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## Basic geometry and measurement

### Course: Basic geometry and measurement>Unit 13

Lesson 1: Constructing triangles

# Construct a right isosceles triangle

Can you build a triangle that is both a right triangle and an isosceles triangle? Created by Sal Khan.

## Want to join the conversation?

• unique triangle.. what about reflected triangles? they would have the same lengths!
• That would be the same triangle. The question is asking, can you make another triangle that has at least one different angle or side length.
• If you had a quadrilateral for which you knew all 4 sides, have you uniquely determined it? Are there other quadrilaterals with the same 4 sides but different angles?
• Nice question and the answer is that there can be other quadrilaterals with same side lengths but different angles. For example, both squares and rhombi have equal side lengths for all 4 sides, but a square is restricted to only have right angles while a rhombus is not.
• I am having a lot of trouble on this topic... could you please add maybe another problem in this video?
• you can use a pro tracker to measure the angles,,,thats what i use
• How do i make a right triangle wth sides 5,8,and 10
• Turn the volume up 💀
(1 vote)
• Can someone explain me one a few things here because I didn't really understand much. like what if you need to make a 80 degree triangle with a 9 inch side? how am I supposed to find out if it even can be made or if its not a unique triangle? I'm bad at making triangles for math so I need some help to figure out how to know. I feel like this is a stupid question. (T-T)
• If it is a right isosceles triangle, you would first make the 90 degree angle. Then you would drag the other two points until the side across from the 90 degree angle is 9 inches and the other two sides are equal.
If you are making an isosceles triangle with just a 80 degree corner and no 90, then you would first make the 9 inch side, then drag the corner across from it until it is 80 degrees, then make the last two sides of the triangle equal.
Hope this helps!
• do i use a protractor for some questions on constucting triangles if not can you give me a step by step instuction to solve a problem
-thank you
• Wait didn't you just make one at ?
• A Shape has 3 equal sides that are each 4 cm in length right angle. How can draw best shape?
• i still cant perfect the triangle i keep messing up any advice ?
(1 vote)
• Triangles can be very challenging, but like everything else if you have a good understanding of the basics you can often work them out.
Try going back to the shape questions for year k through to year 8. Many primary teachers skip over teaching shapes quickly by focusing on vertices, faces, edges, labeling common shapes, 2d and 3d shapes' perimeter, area and volume and when it comes to angles as long as you can use a protractor you can work out the rest.
The integration and extension of shapes and angles together is missed due to lack of teacher interest or more frequently lack of time.
Finding and filling gaps in a person's knowledge then falls to the individual to take responsibility for and with math on Kahn academy you have all you need to go right back to the start.
Enjoy the journey.

## Video transcript

They're asking us to draw a right triangle. So that means it has to have a 90-degree angle. But it's also an isosceles triangle, so that means it has to have at least two sides equal and has two sides of length 3. So those two sides that are going to be equal are going to be of length 3, and it's got to be a right triangle. So let's see if we can do that. So let's try to make this right over here the right angle. And let's make this side and this side have length 3, so 3 and then 3 right over there. Let me make sure I get that right angle right. OK, there you go. So it's a right angle. It's isosceles. At least two sides are equal. And the two sides have length 3. So it seems like we've met all of our constraints. Now they say, is there a unique triangle that satisfies this condition? So another way of rephrasing that, is this the only triangle that I could have drawn that meets these conditions? Well. I can't change this angle if I want to meet these conditions. I can't change these two lengths. And if you keep this angle constant and you keep these two lengths constant, then this point and this point are going to be there are no matter what. So this is the only side that can connect those two points. So this is the only triangle that meets those conditions. You can't have different side lengths, or you couldn't have different angles right over here and also meet those conditions. So is there a unique triangle that satisfies the given conditions? Yes, there's only one unique triangle.