Main content

## High school geometry

### Course: High school geometry > Unit 3

Lesson 5: Working with triangles# Isosceles & equilateral triangles problems

CCSS.Math: ,

Isosceles triangles have two congruent sides and two congruent base angles. Equilateral triangles have all side lengths equal and all angle measures equal. We use these properties to find missing angles in composite figures. The problems are partly from Art of Problem Solving, by Richard Rusczyk. Created by Sal Khan.

## Want to join the conversation?

- What are conjugate angles?(24 votes)
- Conjugate angles are a pair of angles that add up to 360 degrees. E.g. 60 degrees and 300 degrees.(11 votes)

- How to prove a 30, 60, 90 degrees angle triangle's hypotenuse is always two times the side that opposite the 30° angle?(8 votes)
- Nice question!

Draw a 30-60-90 triangle and its reflection about the leg opposite the 60° angle. These two 30-60-90 triangles together form a larger triangle. This larger triangle has three 60° angles and is therefore equilateral!

The hypotenuse of either one of the 30-60-90 triangles is one of the sides of the equilateral triangle. The sides opposite the 30° angles of the two 30-60-90 triangles are equal in length, and the two of them together form another side of the equilateral triangle.

It then follows that the length of the side opposite the 30° angle of a 30-60-90 triangle is half the length of the hypotenuse!

Have a blessed, wonderful day!(26 votes)

- How can you remember the difference between equilateral and isosceles triangles?(5 votes)
- Hmmm....well, equilateral is
**equal**all around that easy.

Isosceles is a bit harder; you could remember that there is one**isolated**side that’s not like the other two. Since isolated sounds like isosceles a little.

That’s all I can think of right now.

Merry Christmas and Jesus loves you❤️(16 votes)

- I know complementary angles add up to 90 degrees and supplementary angles add up to 180 degrees but is there a word for angles that add up to 360 degrees?(4 votes)
- wow its been 10 years since you posted this thats when i was 4 years old(6 votes)

- huh? I still do not understand this😅(4 votes)
- What part? Finding the angles in general? Finding supplementary? Well, I don't know which one you need help with, but here is some tips:

"Straight" for Supplementary (Because it is straight,**180 degrees**)

"Corner" for Complementary (Because it makes a corner,**90 degrees**)

All of a triangle's angles add up to**180 degrees**, but not squares, though! Square's angles add up to**360 degrees**, if that make since? Half of a square is a triangle, so that is correct. :3

If you need more help, feel free to ask for help!

-Duskpin, the avatar(10 votes)

- Why does a triangle add up to 180 ?(5 votes)
- Because that's the only solution for triangles. The sum of the angles will always be 180 degrees.(5 votes)

- How do you remember the difference between supplementary and complimentary angles?(6 votes)
- I hate to break it to you, but you're just going to have to memorize it. But fear not, after practicing for about a week, I'm sure you'll know it!

Good luck!(4 votes)

- If the expressions are the equal legs of an isosceles, or equilateral triangle, then we should go

• Create an equation with the equivalent expressions, by setting them equal to each other,

then solve for x using algebraic methods, (by keeping the equation balanced by performing the same math operations on both sides), to…

• Isolate x to one side of the equation am I right?(5 votes)- If I understand your question right, yes you are.

Hope this helps!(5 votes)

- Through time stamps4:55and5:04Sal states that angle ABE would be 105 degrees. wouldn't it be 210 degrees as angle "e" is 45 degrees and angle b is 105 degrees and angle "a" is 60 degrees? 60+105+45=210 right? so it should be 210 degrees or am i mistaken?(4 votes)
- You seem to gap a little gap in your understanding of angle names. You might have thought that angle ABE was the sum of angles A, B, and E, but in reality, it is the angle with vertex B and two rays, of which one intersects point A and the other point E.(5 votes)

- At2:15Khan said that "the base angles are going to be congruent". But why though?(4 votes)
- That is pretty much part of being an isosceles triangle. If two sides are congruent, the two angles across from them (known as base angles) have to be congruent also.(3 votes)

## Video transcript

Let's do some example
problems using our newly acquired knowledge
of isosceles and equilateral triangles. So over here, I have kind of
a triangle within a triangle. And we need to figure out this
orange angle right over here and this blue angle
right over here. And we know that
side AB or segment AB is equal to segment BC,
which is equal to segment CD. Or we could also call that DC. So first of all, we see that
triangle ABC is isosceles. And because it's isosceles,
the two base angles are going to be congruent. This is one leg. This is the other
leg right over there. So the two base angles
are going to be congruent. So we know that this angle right
over here is also 31 degrees. Well, if we know two of
the angles in a triangle, we can always figure
out the third angle. They have to add
up to 180 degrees. So we could say 31 degrees plus
31 degrees plus the measure of angle ABC is
equal to 180 degrees. You can subtract 62. This right here is 62 degrees. You subtract 62 from both sides. You get the measure of angle
ABC is equal to-- let's see. 180 minus 60 would be 120. You subtract another 2. You get 118 degrees. So this angle right over
here is 118 degrees. Let me just write it like this. This is 118 degrees. Well, this angle
right over here is supplementary to
that 118 degrees. So that angle plus 118 is
going to be equal to 180. We already know that
that's 62 degrees. 62 plus 118 is 180. So this right over
here is 62 degrees. Now, this angle is one of the
base angles for triangle BCD. I didn't draw it that way,
but this side and this side are congruent. BC has the same length as CD. Those are the two legs
of an isosceles triangle. You can kind of imagine
it was turned upside down. This is the vertex. This is one base angle. This is the other base angle. Well, the base angles are
going to be congruent. So this is going to be
62 degrees, as well. And then finally, if you want
to figure out this blue angle, the blue angle plus these
two 62-degree angles are going to have to
add up to 180 degrees. So you get 62 plus 62
plus the blue angle, which is the measure of
angle BCD, is going to have to be equal
to 180 degrees. These two characters--
let's see. 62 plus 62 is 124. You subtract 124
from both sides. You get the measure of angle
BCD is equal to-- let's see. If you subtract 120,
you get 60, and then you have to subtract another 4. So you get 56 degrees. So this is equal to 56 degrees. And we're done. Now, we could do
either of these. Let's do this one
right over here. So what is the
measure of angle ABE? So they haven't even
drawn segment BE here. So let me draw that for us. And so we have to figure out
the measure of angle ABE. So we have a bunch of
congruent segments here. And in particular, we see that
triangle ABD, all of its sides are equal. So it's an equilateral triangle,
which means all of the angles are equal. And if all of the angles
are equal in a triangle, they all have to be 60 degrees. So all of these characters
are going to be 60 degrees. Well, that's part
of angle ABE, but we have to figure out this
other part right over here. And to do that, we can
see that we're actually dealing with an
isosceles triangle kind of tipped over to the left. This is the vertex angle. This is one base angle. This is the other base angle. And the vertex angle
right here is 90 degrees. And once again, we
know it's isosceles because this side, segment
BD, is equal to segment DE. And once again, these two angles
plus this angle right over here are going to have to
add up to 180 degrees. So you call that an x. You call that an x. You've got x plus x plus 90
is going to be 180 degrees. So you get 2x plus-- let
me just write it out. Don't want to skip steps here. We have x plus x
plus 90 is going to be equal to 180 degrees. x plus x is the same thing as
2x, plus 90 is equal to 180. And then we can subtract
90 from both sides. You get 2x is equal to 90. Or divide both sides by 2. You get x is equal
to 45 degrees. And then we're done
because angle ABE is going to be equal to the 60
degrees plus the 45 degrees. So it's going to be
this whole angle, which is what we care about. Angle ABE is going to be 60
plus 45, which is 105 degrees. And now we have this
last problem over here. This one looks a
little bit simpler. I have an isosceles triangle. This leg is equal to that leg. This is the vertex angle. I have to figure out B. And the trick here is like,
wait, how do I figure out one side of a triangle if
I only know one other side? Don't I need to know
two other sides? And we'll do it
the exact same way we just did that second
part of that problem. If this is an isosceles
triangle, which we know it is, then this angle is going to
be equal to that angle there. And so if we call this x,
then this is x as well. And we get x plus x plus
36 degrees is equal to 180. The two x's, when you
add them up, you get 2x. And then-- I won't
skip steps here. 2x plus 36 is equal to 180. Subtract 36 from both
sides, we get 2x-- that 2 looks a little bit funny. We get 2x is equal to--
180 minus 30 is 150. And then you want to
subtract another 6 from 150, gets us to 144. Did I do that right? 180 minus 30 is 150, yep, 144. Divide both sides by 2. You get x is equal
to 72 degrees. So this is equal to 72 degrees. And we are done.