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High school geometry
Course: High school geometry > Unit 5
Lesson 3: Special right triangles30-60-90 triangle example problem
Using what we know about 30-60-90 triangles to solve what at first seems to be a challenging problem. Created by Sal Khan.
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In the previous 30-60-90 video I thought I understood the ratio of the angles to be x: square root of 3 over 2 times x : and x over 2. At2:40Sal says the ratio is 1: square root of 3 : 2. Can someone explain this please?(20 votes)- They're the same thing, it's just that the former is expressed in terms of x (which is the hypotenuse). But first, you have to be careful: since we're only dealing with ratios, you can order the three sides as you please, but it's better to order from shortest to longest side, otherwise it might get confusing. So here, it would be better to say x/2 : (sqrt(3)/2)x : x. This is consistent with 1 : sqrt(3) : 2, shortest to longest. Also, you might or might not be a bit confused by the notation I used, but I'm sure if you compare it with what you know about the ratios, you'll be fine. But I'd suggest you to be careful with how you write mathematical expressions; you wrote "square root of 3 over 2 times x", but this is actually very ambiguous and can be interpreted in at least two ways, one of which means something pretty different from what you want!
Anyway, as I said, they're the same. If you take a closer look at 1 : sqrt(3) : 2, you'll notice that the length of the longest side, which is 2, is two times that of the shortest side, which is 1. So if you say the longest side is x, then the shortest side is x/2. Also, we know that the ratio of the shortest side to the middle one is 1 : sqrt(3). In other words, you have to multiply the shortest side by sqrt(3) in order to get the middle side. So:
sqrt(3) * x/2 = (sqrt(3)/2)x. You could also write is sqrt(3)*x/2.
But anyway, yeah, 1 : sqrt(3) : 2, and x/2 : (sqrt(3)/2)x2 : x are the exact same thing!(21 votes)
Is it not true that the middle Δ BDE is an Isoceles triangle? Since Angle EBD= 30 deg.
Then why not use the 2/√3 is the measure of ED. Adding 2/√3 +2/√3 +2 = 4/√3 + 2 = perimeter of triangle BDE.(17 votes)
I got the same answer. I suppose the exercise is flawed in that way...?(5 votes)
At2:15, Sal says that the 60-degree side is going to be √3 times 1, but I don't understand how he got √3.(8 votes)
The 30-60-90 ratio states that if the side across from 30* angle is x, then the side across from 60 will be x*√3 and the one across from the 90* will be 2x. Therefore, if x is one, then the side across from 60 will be 1*√3 = √3(9 votes)
I still don't get how Sal got a square root of 3/3 from 1/ square root of 3.(4 votes)
Mathematicians do not like radicals in the bottom, so if we start from 1/√3, we can multiply by √3/√3 (this is just 1) to get (1*√3)/(√3*√3). Since √3*√3=√9=3, we end up with √3/3.(13 votes)
I don't understand how Sal got a side length measured to the square root of 3 at2:13Anyone?(4 votes)
That's a relationship 30, 60, 90 triangles have. Until you learn trig relations it's just something you have to memorize(3 votes)
- At the very end, the perimeter was 1/sqrt3 + sqrt3 + 2, then you multiplied by sqrt3/sqrt3 (1) to make 1/sqrt3 into sqrt3 / 3. So with the rest of the numbers, how did sqrt3 become 3 times sqrt3 / 3, what did you multiply the sqrt 3 by?(4 votes)
Try simplifying 3(sqrt3)/3 and see what you had to do(2 votes)
At1:12, how can we know that side DC is equal to 1 if we only know that side AB is equal to 1 in quadrilateral ADCB?(3 votes)
We know that the quadrilateral has 4 right angles, meaning that it must be a rectangle or a square. Squares and rectangles both have the property that two opposite sides must have the same length (But squares have the additional element of all sides having the same length).
In this problem, we already established that it is either a square or a rectangle. Since we know side AB has a length of 1, and that side DC is opposite of side AB, we also know that side DC has a length of 1. We also know that side AD will have the same length as side BC, using the same property of rectangles.(2 votes)
- I am having trouble understanding how to solve a problem if you're given that the hypotenuse is x and the triangle is a 30-60-90 triangle and the side adjacent to the 30 degree angle is 15 units long. How can I find the hypotenuse? Is there a video on this?(2 votes)
The ratio of the side lengths of a 30-60-90 triangle is 1 ∶ √3 ∶ 2
This means that if the shortest side, i.e., the side adjacent to the 60° angle, is of length 𝑎,
then the length of the side adjacent to the 30° angle is 𝑎√3,
and the length of the hypotenuse is 2𝑎
In this case we have 𝑎√3 = 15 ⇒ 𝑎 = 5√3
Thereby the length of the hypotenuse is 2 ∙ 5√3 = 10√3 ≈ 17.3 units(3 votes)
Since the triangle BED is isoceles, and side BE is 2/√3, doesn't side ED have to be 2/√3 as well?(3 votes)- yeah, Sal said side ED is sqrt(3)-1/sqrt(3), which is actually the same as 2/sqrt(3).
sqrt(3) is the same as sqrt(3)*sqrt(3)/sqrt(3)=(sqrt(3)*sqrt(3))/sqrt(3)=3/sqrt(3)
then 3/sqrt(3)-1/sqrt(3)=2/sqrt(3) because they have a common denominator.(2 votes)
Is trisect the same as bisect?(1 vote)- tri (like tricycle) has to do with 3, so trisect is to cut into three equal pieces, bi (like bicycle) has to do with 2, so cut into two equal pieces. They are similar, but not the same.(5 votes)
2:40
Video transcript
So we have this rectangle
right over here, and we're told that the
length of AB is equal to 1. So that's labeled
right over there. AB is equal to 1. And then they tell us that
BE and BD trisect angle ABC. So BE and BD trisect angle ABC. So trisect means dividing
it into 3 equal angles. So that means that this
angle is equal to this angle is equal to that angle. And what they want
us to figure out is, what is the perimeter
of triangle BED? So it's kind of
this middle triangle in the rectangle
right over here. So at first this seems
like a pretty hard problem, because you're like well, what
is the width of this rectangle. How can I even start on this? They've only given
us one side here. But they've actually given
us a lot of information, given that we do know
this is a rectangle. We have four sides, and
that we have four angles. The sides are all
parallel to each other and that the angles
are all 90 degrees. Which is more than
enough information to know that this is
definitely a rectangle. And so one thing we do
know is that opposite sides of a rectangle are
the same length. So if this side is 1, then
this side right over there is also 1. The other thing we know is
that this angle is trisected. Now we know what the
measure of this angle is. It was a right angle, it
was a 90 degree angle. So if it's divided into three
equal parts, that tells us that this angle right
over here is 30 degrees, this angle right over
here is 30 degrees, and then this angle right
over here is 30 degrees. And then we see
that we're dealing with a couple of
30-60-90 triangles. This one is 30, 90, so this
other side right over here needs to be 60 degrees. This triangle right over here,
you have 30, you have 90, so this one has
to be 60 degrees. They have to add up to
180, 30-60-90 triangle. And you can also figure out
the measures of this triangle, although it's not going
to be a right triangle. But knowing what we know
about 30-60-90 triangles, if we just have
one side of them, we can actually figure
out the other sides. So for example, here we
have the shortest side. We have the side opposite
of the 30 degree side. Now, if the 30 degree side
is 1, then the 60 degree side is going to be square
root of 3 times that. So this length
right over here is going to be square root of 3. And that's pretty useful
because we now just figured out the length of the entire
base of this rectangle right over there. And we just used our knowledge
of 30-60-90 triangles. If that was a little
bit mysterious, how I came up with
that, I encourage you to watch that video. We know that 30-60-90
triangles, their sides are in the ratio of 1 to
square root of 3 to 2. So this is 1, this
is a 30 degree side, this is going to be square
root of 3 times that. And the hypotenuse
right over here is going to be 2 times that. So this length
right over here is going to be 2 times this
side right over here. So 2 times 1 is just 2. So that's pretty interesting. Let's see if we can
do something similar with this side right over here. Here the 1 is not the side
opposite the 30 degree side. Here the 1 is the side
opposite the 60 degree side. So once again, if we
multiply this side times square root of 3, we
should get this side right over here. This is the 60, remember this
1, this is the 60 degree side. So this has to be 1 square
root of 3 of this side. Let me write this down, 1
over the square root of 3. And the whole reason, the
way I was able to get this is, well, whatever this
side, if I multiply it by the square root of 3, I
should get this side right over here. I should get the 60
degree side, the side opposite the 60 degree angle. Or if I take the 60 degree
side, if I divide it by the square root of 3 I should
get the shortest side, the 30 degree side. So if I start with the
60 degree side, divide by the square root of 3, I
get that right over there. And then the
hypotenuse is always going to be twice the side
opposite the 30 degree angle. So this is the side opposite
the 30 degree angle. The hypotenuse is
always twice that. So this is the side opposite
the 30 degree angle. The hypotenuse is
going to be twice that. It is going to be 2 over
the square root of 3. So we're doing pretty good. We have to figure
out the perimeter of this inner triangle
right over here. We already figured
out one length is 2. We figured out another length
is 2 square roots of 3. And then all we have to really
figure out is, what ED is. And we can do that
because we know that AD is going to be
the same thing as BC. We know that this entire
length, because we're dealing with a rectangle,
is the square root of 3. If that entire length
is square root of 3, if this AE is 1 over
the square root of 3, then this length
right over here, ED is going to be
square root of 3 minus 1 over the square root of 3. That length minus that
length right over there. And how to find the perimeter
is pretty straight forward. We just have to add these
things up and simplify it. So it's going to be,
just let me write this, perimeter
of triangle BED is equal to-- This is
short for perimeter. I just didn't feel like
writing the whole word.-- is equal to 2 over the square
root of 3 plus square root of 3 minus 1 over the square
root of 3 plus 2. And now this just boils down
to simplifying radicals. You could take a
calculator out and get some type of decimal
approximation for it. Let's see, if we have 2 square
root of 3 minus 1 square root of 3, that will leave us with
1 over the square root of 3. 2 over the square of 3 minus
1 over the square root of 3 is 1 over the square root of 3. And then you have the
square root of 3 plus 2. And let's see, I can
rationalize this. If I multiply the numerator
and the denominator by the square root of 3,
this gives me the square root of 3 over 3 plus the
square root of 3, which I could rewrite
that as plus 3 square roots of 3 over 3. Right? I just multiplied this
times 3 over 3 plus 2. And so this gives us-- this
is the drum roll part now-- so one square root of 3
plus 3 square roots of 3, and all of that over 3, gives
us 4 square roots of 3 over 3 plus 2. Or you could put the 2 first. Some people like to write
the non-irrational part before the irrational part. But we're done. We figured out the perimeter. We figured out the perimeter
of this inner triangle BED, right there.