In this video, we are going to introduce the law of sines and then see how to
apply it to some mixed problems. So this is what the law of sines is all about. It’s really useful because it
enables us to do trigonometry and calculate lengths and angles in triangles that are not right
So I have here a diagram of a triangle which has no right angle in it, and I’ve
labelled it in a particular way. I’ve labelled the three vertices of the triangle as 𝐴, 𝐵, 𝐶 in
capitals and then I’ve labelled the side opposite those with the same letter but in lowercase,
so side 𝑎 is opposite angle 𝐴 and so on.
What the law of sines tells ask is that the ratio between the length of a side
and the sine of its opposite angle is constant within a particular triangle. So if I take side 𝑎 and then I divide it by sine of the opposite angle 𝐴, I get
the same result as if I take side 𝑏 and then divided by sine of angle 𝐵, and I also get the
same result if I take side 𝑐 and then divide it by sine of the opposite angle 𝐶.
Now this is one way of specifying the relationship and this format is
particularly useful if we’re looking to calculate the length of a side. But you can also
specify it using the reciprocals, so I can invert each of those fractions. So it can also be written in this format here, where each of those fractions is
just the other way up. This format is particularly useful if you’re being asked to calculate
the size of an angle.
So when do we use the law of sines? Well as I’ve already said, we use it in non-right
-angled triangles, but more specifically we use it when the information we’re given on
what we want to work out is made up of opposite pairs. So for example, I might know the lengths of sides a and 𝑏 and I might know angle
𝐴 and want to calculate angle 𝐵. And so because those are opposite pairs, this would be the
opportunity to use the law of sines.
This question says find all the possible values for the other lengths and angles
in triangle 𝐴𝐵𝐶, and we’re asked to give lengths to the nearest centimetre and angles to the
nearest degree. So we’ll discuss what is meant by all the possible values a little bit later, but
let’s start off by recalling the definition of the law of sines that we’re going to need
within this question.
And remember it was this ratio here, and of course we could have the reciprocal
of that so we could have it written the other way up. So I have three things that I need to calculate here: two missing angles and then
one missing length. The reason I know that I can use the sine ratio is because I have opposite
So I have that length fourteen centimetres and the angle fifty-two degrees and
then I have this length eight point one centimetres, which means I have enough information to
calculate angle 𝐵 first of all. So what I’m gonna do is I’m going to write down the law of sines just using
angle 𝐴, side 𝑎, angle 𝐵, and side 𝑏.
Now as I’m calculating an angle first of all, I’m gonna use the reciprocal form
of that relationship. So using all the known information, I have that sine of angle 𝐵 divided by eight
point one is equal to sine of fifty-two divided by fourteen. And now I have an equation that I
can solve in order to work out angle 𝐵. The first step is I’m gonna multiply both sides of this
equation by eight point one.
So I have that sine of 𝐵 is equal to eight point one multiplied by sin
fifty-two over fourteen. Now I’m gonna use the inverse sine function in order to calculate
angle 𝐵. And then using my calculator to evaluate this, it tells me that angle 𝐵 is equal
to twenty-seven point one two four. I’m gonna round that then to twenty-seven degrees.
So if I have angle 𝐵 is twenty-seven degrees and I already know angle 𝐴 is
fifty-two degrees, I can work out angle 𝐶 straight away not using the law of sines but just
using the angle sum in a triangle. So I have angle 𝐶 is one hundred and eighty subtract fifty-two subtract
twenty-seven, and therefore it’s one hundred and one degrees.
So now I have all the angles in the triangle; I just need to work out the final
side. I’m going to apply the law of sines again then. And as I’m calculating the length of a
side this time, I’m going to use the first version where the sides are in the numerator. Now I only need to use one of the other pairs, so either pair A or pair B. I’m going
to choose to use pair A.
So I have that 𝑐 over sin one hundred and one is equal to fourteen over sin
fifty-two. Now be careful to distinguish between lowercase and uppercase letters here. Remember
lower case letters represent sides, so that is a lowercase letter 𝑐. To solve this equation for side 𝑐 then, I need to multiply both sides of the
equation by sin one hundred and one.
And I have then that 𝑐 is equal to fourteen multiplied by sine of a hundred and
one divided by sine of fifty-two, which is seventeen point four three. Now I’m asked to round
this to the nearest centimetre, so then I have seventeen centimetres for side 𝑐. So those three calculated values together then, I have that side 𝑐 is seventeen
centimetres, angle 𝐵 is twenty-seven degrees, and angle 𝐶 is a hundred and one degrees.
Now let’s come back to that part of the question where it asked us for all the
possible values for the other lengths and angles. And what we need to consider is when we
worked out angle 𝐵, we saw that it was equal to twenty-seven degrees. What we need to consider is that there is in fact another possibility for angle
𝐵 which uses the fact that sine of an angle is equal to sine of a hundred and eighty minus
That’s just one of the properties of the sine ratio. So what this tells us is
that although angle 𝐵 could be twenty-seven degrees, it could also be one hundred and eighty
minus twenty-seven, which would mean that 𝐵 could be a hundred and fifty-three degrees. However, if we
look at the information that we already had at this stage, which was that angle 𝐴 was fifty-two
degrees, that doesn’t work. Because if we add 𝐴 and 𝐵 together, that would take the angle sum
above a hundred and eighty degrees, and we know that in a triangle a hundred and eighty degrees
is the sum of the angles.
This tells us that in fact there isn’t another possibility for angle 𝐵. Because
if it were a hundred and fifty-three degrees, it wouldn’t be possible to incorporate that in a
triangle with the information we already know. This is an important check though. And if it had been the case that this angle
could be included in a triangle with the fifty-two degrees, then we would have another set of
possibilities and we would need to go through the process of calculating angle 𝐶 and side 𝑐
again using the second value of 𝐵.
So in this question, we’ve applied the law of sines twice, once to calculate a
side and once to calculate an angle. And then we’ve used the fact that angles in a triangle sum
to a hundred and eighty degrees in order to find the third angle in the triangle.
This question tells us that 𝐴𝐵𝐶 is a triangle, angle 𝐴 is fifty-five degrees, 𝐵𝐶
is thirteen centimetres, and 𝐴𝐶 is twenty-eight centimetres. We’re asked if the triangle exists,
find all the possible values for the other lengths and angles and then we’re told how to round
our answers. Now it’s interesting that the question says if the triangle exists. So what we’ll
do is we’ll assume the triangle does exist, and we’ll go through all the working out. And if it
works perfectly fine, then the triangle does exist. And if we come up against a problem, then
we’ll see that the triangle doesn’t exist.
So I’m gonna assume it exists first of all, and I’m gonna draw a sketch of this
triangle. So here is my diagram, with all the information that I was given put onto it.
Now we’re asked to calculate lengths and angles, and I can recognise that I need
to use the law of sines here because I’ve got an opposite pair there of fifty-five degrees and
thirteen centimetres. So let’s recall the law of sines. And it’s this here: that the ratio between the sine of an angle and the length of
the opposite side is constant throughout the triangle. Remember, lowercase 𝑎, 𝑏, and 𝑐 there are
representing the sides opposite angles 𝐴, 𝐵, and 𝐶.
So I just include them on the diagram there. Now I’ve chosen to use the law of
sines in this format, where the angles are in the numerator because I’m gonna try and calculate
angle 𝐵 first of all. And it just requires less rearranging if I start off with the angle being
in the numerator. I could use the other version with the reciprocals of each these fraction, but
it would just require a slightly more complicated rearrangement.
So what I’m going to do then is I’m gonna write out this law of sines using
the pair that I know so that’s pair A and using the pair I want to calculate which is pair
B. And I have then that sine of angle 𝐵 divided by twenty-eight is equal to sine of
fifty-five divided by thirteen.
So this gives me an equation that I’m looking to solve then to work out angle 𝐵.
And the first step is to multiply both sides of this equation by twenty-eight. In doing so, I get that sine of 𝐵 is equal to twenty-eight sin fifty-five over
thirteen. Now in order to work out angle 𝐵, I need to use the inverse sine function.
And I have then that 𝐵 is equal to the inverse sine of this ratio: twenty-eight
sin fifty-five over thirteen. Now if you try to type that into your calculator, you’ll see that
you get some kind of error and you can’t actually work out angle 𝐵. Let’s go back to the stage
before to see why this is.
If I actually evaluate the fraction at this stage here, you’ll see that it’s
equal to one point seven six four. So we have sine of 𝐵 is equal to one point seven six four,
and that’s why we get an error. If you recall the value of the sine of any angle is always between negative one
and one. And in the case of a positive angle as we’d have in a triangle, it’s always between
zero and one. And therefore it’s not possible for sine 𝐵 to be equal to this value of one point
seven six, which exceeds one.
This tells us then that we can’t calculate angle 𝐵 and therefore our assumption
that this triangle exists must be false. Our answer to the question then is that we can’t calculate the lengths of any of
these sides or the sizes of any of the angles because the triangle does not exist.
Let’s look at a worded problem. It tells us that James wants to calculate the height
of a tall building. He looks at the building from the same horizontal plane, and the angle of
elevation to the top is forty degrees. James then moves thirty metres further back, and the angle
of elevation is now twenty-five degrees. We’re asked to calculate the height of this building
to the nearest tenth.
So we aren’t given a diagram, and it’s always a sensible idea to draw our own. So we start off with a tall building. James is standing a certain distance away
from it. We don’t know how far. And the angle of elevation to the top is forty degrees. James now moves thirty metres further back, and the angle of elevation is now
twenty-five degrees, so we add this part onto the diagram.
So add some letters onto the diagram, and it’s 𝐵𝐷 that we’re looking to calculate,
the height of the building. Now 𝐵𝐷 is a right-angled triangle, so in theory we can use normal trigonometry — sine,
cosine, and tangent ratios — in this triangle. But we only know one angle at the moment, so we
need some other information, preferably a length, in order to work out the length of 𝐵𝐷.
We also have a non-right-angled triangle, triangle 𝐴𝐵𝐶 in which we have a bit
more information. We know an angle and a side. You’ll also notice that side 𝐴𝐵 is common to both
of these triangles. So perhaps we can use the non-right-angled triangle to work out 𝐴𝐵 and then
use trigonometry in triangle 𝐴𝐵𝐷 in order to find the height of this building.
So let’s look at the non-right-angled triangle, triangle 𝐴𝐵𝐶, first. And actually
we can work out all three of the angles in this triangle because that angle of forty degrees
is sitting on a straight line with the other angle, and therefore this other angle must be one
hundred and forty degrees using the fact that angles on a straight line sum to a hundred and
So this angle here, angle 𝐴, must be a hundred and forty degrees. We can also work
out angle 𝐵 because we know the angle sum in a triangle is a hundred and eighty degrees, so
angle 𝐵 must be fifteen. So looking at this non-right-angled triangle, we know all three angles and we
know one side, which means we can apply the law of sines in order to work out the length of
either of the other two sides.
So I’ll give them the letters lowercase 𝑎, lowercase 𝑏, and lowercase 𝑐
corresponding to the angles they’re opposite. And let’s recall the law of sines. Here it is. I’m using it in this format with the lengths and the numerator
because it’s a length that I’m looking to calculate.
So I’m looking to calculate the length of 𝐴𝐵, which is referred to here as side
𝑐, so I’m gonna use side 𝑐 and angle 𝐶. And I’m also gonna use side 𝑏 and angle 𝐵 because
they’re the two that I know What I have then is that 𝑐 over sin twenty-five is equal to thirty over sin
fifteen using the opposite pairs. I can solve this equation then in order to work out 𝑐; I need
to multiply both sides by sin twenty-five.
This tells me that 𝑐 is equal to thirty sin twenty-five over sin fifteen. And
evaluating that, it tells me it’s forty-eight point nine eight six one. Now I’m going to keep that value on my calculator so that I have it there
exactly to use later in the calculation.
Now if I turn my attention to the right-angled triangle, triangle 𝐴𝐵𝐷, I have the
size of an angle, forty degrees, I have length 𝑐 or 𝐴𝐵, which is forty-eight, and I want to
calculate length of that side 𝐵𝐷. So I can use standard trigonometry. I’m going to begin by
labelling the three sides of that triangle with their labels in relation to that angle of forty.
So I have the opposite, the adjacent, and the hypotenuse. Now I know the
hypotenuse and I want to calculate the opposite. So that tells me it’s the sine ratio that I’m
using, just the standard sine ratio and a right-angled triangle, not the law of sines. The sine ratio, remember, is the opposite divided by the hypotenuse.
Now you can actually use the law of sines in a right-angled triangle but it’s
unnecessarily complicated it involves using that ninety-degree angle and a sine of
ninety is just one. It’s more straightforward to just use the regular sine ratio. So I’m gonna write this ratio out for this triangle. So I’ll have sine of forty
is equal to the opposite 𝐵𝐷 over forty-eight point nine eight.
I want to solve this equation to find the value 𝐵𝐷, so I need to multiply both
sides by that value of forty-eight point nine eight six. That’s why I kept that value on my calculator because now I can just press
multiply by sin forty in order to get an exact answer.
So evaluating that gives me thirty-one point four eight. And then rounding it to
the nearest tenth as requested gives me an answer of thirty-one point five metres for the height
of this building. So in this question, drawing an appropriate diagram first of all is important. We
then use the law of sines in a non-right-angled triangle and then use the normal sine ratio in
a right-angled triangle in order to answer the question.
In summary then, the law of sines allows us to calculate angles and sides in
non-right-angled triangles. It tells us that the ratio between a side and the sine of the
opposite angle is constant throughout the triangle, and we can use it in either of these two
formats depending on whether we’re looking to calculate the length of a side or the size of an