Law of sines Video Lecture - Class 11

Voiceover:We've got a triangle here where
we know two of the angles
and one of the sides.
And what I claim, is that I
can figure out everything else
about this triangle just
with this information.
You give me two angles and a side,
and I can figure out what the other
two sides are going to be.
And I can, of course,
figure out the third angle.
So, let's try to figure that out.
And the way that we're going to do it,
we're going to use something
called the Law of Sines.
In a future video, I will
prove the Law of Sines.
But here, I am just going to show you how
we can actually apply it.
And it's a fairly straightforward idea.
The Law of Sines just tells us
that the ratio between
the sine of an angle,
and the side opposite to it,
is going to be constant for any
of the angles in a triangle.
So for example, for this
triangle right over here.
This is a 30 degree angle,
This is a 45 degree angle.
They have to add up to 180.
So this right over here
has to be a, let's see,
it's going to be 180 minus 45 minus 30.
That's 180 minus 75, so
this is going to equal
105 degree angle, right over here.
And so applying the Law of Sines,
actually let me label the different sides.
Let's call this side right over here,
side A or has length A.
And let's call this side,
right over here, has length B.
So the Law of Sines tells us
that the ratio between
the sine of an angle,
and that the opposite side is going to be
constant through this triangle.
So it tells us that sine of
this angle, sine of 30 degrees
over the length of the side opposite,
is going to be equal to
sine of a 105 degrees,
over the length of the
side opposite to it.
Which is going to be equal
to sine of 45 degrees.
equal to the length of the side opposite.
So sine of 45 degrees over B.
And so if we wanted to figure out A,
we could solve this
equation right over here.
And if we wanted to solve for B,
we could just set this equal
to that right over there.
So let's solve each of these.
So what is the sine of 30 degrees?
Well, you might just remember
it from your unit circles
or from even 30, 60, 90
triangles and that's 1/2.
And if you don't remember it,
you can use a calculator to verify that.
I have already verified that
this is in degree mode, so it's 0.5.
So this is going to be
equal to 1/2 over two.
So another way of thinking about it,
that's going to be equal to 1/4,
this piece is equal to 1/4
is equal to sine of a 105 degrees over A.
Let me write this, this is equal to
sine of 105 degrees over A.
And actually, we could also say,
since we could actually
do both at the same time,
that this is equal to that.
That 1/4 is equal to sine
of 45 degrees over B.
Actually, sine of 45 degrees
is another one of those
that is easy to jump out of unit circles.
You might remember it's
square of two over two.
Let's just write, that's
square root of two over two.
And you can use a calculator,
but you'll get some decimal
value right over there.
But either case, in
either of these equations,
let's solve for A then let's solve for B.
So one thing we could do is we could
take the reciprocal of both
sides of this equation.
The reciprocal of 1/4 is four.
And the reciprocal of this right-hand side
is A over the sine of 105 degrees.
And then to solve for A,
we could just multiply
both sides times the
sine of a 105 degrees.
So we get four times the sine
of 105 degrees is equal to A.
Let's get our calculator out,
so four times the sine of 105 gives us,
it's approximately equal to,
let's just round to the
nearest 100th, 3.86.
So A is approximately equal to 3.86.
Which looks about right if this is two,
and I have made my angles appropriately,
that looks like about 3.86.
Let's figure out what B is.
We could once again take the reciprocal of
both sides of this and we get
four is equal to B over
square root of two over two,
we could multiply both sides times
square root of two over two.
And we would get B is equal to
four times the square
root of two over two.
Come to think of it, B is four times
the sine of 45 degrees.
Let's figure out what that is.
If we wanted actual numerical value,
we could just write this
as two square roots of two.
But let's actually
figure out what that is.
Two square roots of two is equal to 2.83.
So B is approximately equal to 2.83.
So [I'm] be clear, this
four divided by two is
two square roots of two, which is 2.8.
Which is approximately equal to 2.83
if we round to the nearest 100th, 2.83,
which also seems pretty reasonable here.
So the key of the Law of Cosines is
if you have two angles and a side,
you're able to figure out
everything else about it.
Or if you actually had
two sides and an angle,
you also would be able to figure out
everything else about the triangle.