### Video Transcript

In this video, we’re going to see how the three trigonometric ratios sine, cosine, and
tangent can be applied to a mixed set of problems.

So a reminder then of what the trigonometric ratios are. I have here a diagram of a
right-angled triangle, and I’ve labeled one of the other angles as 𝜃. I’ve then labeled
the three sides of the right-angled triangle in relation to that angle 𝜃. So we have
the opposite, the adjacent, and the hypotenuse.

The trigonometric ratios are the ratios that exist between different pairs of sides
within this triangle. So we have their definitions on the screen here. Firstly, sine of
𝜃 is the ratio between the opposite and the hypotenuse. So it’s opposite divided by
hypotenuse. Cosine of 𝜃, which is usually abbreviated to cos, is the adjacent divided
by the hypotenuse. And finally, tangent of 𝜃, usually abbreviated to tan, is the
opposite divided by the adjacent.

So we’ll now look at a mixed set of problems where these ratios will be needed. And
we’ll see how to identify which ratio it is that’s required and then how to apply it.

This problem says, a palm tree 10.6 meters tall is observed from a point 12 meters away
on the same horizontal plane as the base of the tree. We’re asked to find the angle of
elevation to the top of the palm tree and then give our answer to the nearest minute.

So to start off with, we haven’t been given a diagram in this question. And I would
always suggest that if you’re not given a diagram, then you draw your own. So we’re going
to draw a sketch of the palm tree, the horizontal ground, and then the line of sight
from the ground to the top of the palm tree.

So this of course forms a right-angled triangle. Now we need to put some information onto
this diagram. We’re told the palm tree 10.6 meters tall. And we’re also told that the
observer is standing 12 meters away.

So we have these two sides of the right-angled triangle. The angle of elevation, remember,
is measured from the horizontal to the top of the palm tree. And therefore it is this
angle here that we’re looking to calculate.

So for any problem involving trigonometry, it’s always a good idea once you’ve got the
diagram to then label the three sides of the triangle. So in relation to this angle 𝜃,
I’m going to give them their labels of the opposite, the adjacent, and the hypotenuse.
And once I’ve done that, I can see that the two lengths I have are the lengths of the
opposite and the adjacent sides.

Now this tells me that it’s the tan ratio that I’m going to be using in order to
calculate this angle. Because if you think back to SOHCAHTOA, then TOA is where the
opposite and the adjacent appear together.

So I recall the definition of the tan ratio. And then I need to write it down using the
information in this question. So I’m gonna replace the opposite with 10.6 and the
adjacent with 12.

I have then that tan of 𝜃 is equal to 10.6 over 12. Now I’m looking to calculate that angle
𝜃. So I need to use the inverse tan function here. So I have then that 𝜃 is equal to
tan inverse of this ratio, 10.6 over 12.

Now I can use my calculator in order to evaluate that, making sure my calculator is in
degree mode. And this tells me that 𝜃 is equal to 41.4552. Now this answer is in
degrees. And I’ve been asked in the question to give my answer to the nearest minute. So
I now need to convert this from degrees into degrees and minutes.

So what I can see then is that I have 41 full degrees. And then I also have this decimal
0.4552 and so on left over. And that’s the part that I need to convert into minutes.

Now remember that a minute is one sixtieth of a degree. So in order to work out what
this decimal represents in terms of minutes, I need to multiply it by 60. When I do so,
this gives me an answer of 27.3140. So rounding that to the nearest minute, I have 27
minutes.

Finally, I need to pull these two parts of my answer together. And doing so tells me
that this angle of elevation is 41 degrees 27 minutes to the nearest minute.

So in this question then, we drew a diagram using the information that we were given. We
identified the need for the tangent ratio because we had the opposite and adjacent
sides. We then used the inverse tangent function in order to work out the sides of this
angle, finally converting our answer into degrees and minutes.

This problem is about a circle. It tells us that a chord is drawn in a circle whose
radius is eight centimeters. If the size of its central angle is 100 degrees, we’re
asked to calculate the length of the chord to the nearest thousandth.

So as I haven’t been given a diagram, I’m gonna start off by drawing my own. And I’m
gonna begin with a circle. Now it tells me that a chord is drawn in the circle. So that
is a line connecting two points on the circumference but not passing through the center
of the circle.

So here is my chord. And then it tells me that the central angle is a 100 degrees. So
I’m gonna connect the two ends of the chord to the center of the circle. And that’s going
to create an angle of 100 degrees.

Now there’s some other information we know, which is that the radius of the circle is
eight centimeters. So those two lines connecting the center of the circle to the ends of
the chord are both eight centimeters.

So what I can see then is I have an isosceles triangle because this triangle has two
sides of the same length. In order to do trigonometry though, I’d like to have a
right-angled triangle.

So what I’m going to do is I’m gonna draw in the perpendicular height of this isosceles
triangle. And what that does is it divides the triangle up into two congruent; that is,
identical, right-angled triangles. So that’s drawing in this line here.

Now because that line divides the isosceles triangle directly in half, that 100-degree
angle is now split evenly, 50 degrees and 50 degrees, into each of these two
right-angled triangles. So to have you visualize it a little bit better, I’m just gonna
draw out one of these two right-angled triangles.

So here we have then that triangle a little bit bigger. Now I’m asked to calculate the
length of the chord. So what I’m going to do is I’m going to use trigonometry to
calculate the base of the triangle, this side here. And then I’m going to double it in
order to get the full length of the chord.

So the first step then is to label all of the sides of the triangle in relation to this
angle of 50 degrees. So we have the opposite, the adjacent, and the hypotenuse.

So what I can see then is that I’m going to be using the sine ratio because I know the
hypotenuse and I want to calculate the opposite. And so that’s the SOH part of
SOHCAHTOA.

Now I’ll just give the opposite a different letter so that we can save confusion with
zero in our working out. I’ll call it 𝑥 centimeters.

So we recognized the need for the sine ratio. And I’ve recalled its definition here.
It’s that sine of 𝜃 is equal to the opposite divided by the hypotenuse. So I’m gonna
write this ratio out using the information in this question. I’m gonna replace the angle
with 50 degrees, the opposite with 𝑥, and the hypotenuse with eight.

So then here is an equation that I can use in order to work out the value of 𝑥. Now 𝑥
is currently divided by eight. So I’m gonna multiply both sides of this equation by
eight. And I’ve just swapped the two sides of the equation round here. But this tells me
that 𝑥 is equal to eight sin 50.

Now I could evaluate it at this stage here. But actually I’m not asked to find 𝑥.
Remember, I’m asked for the length of the chord. So I need to double this. So what I’m
gonna do is just multiply this by two. So I have that the length of the chord is two
lots of eight sin 50, which is just equivalent to 16 sin 50. And now I’m going to use my
calculator to evaluate this.

And it tells me that this is equal to 12.2567. Now the question asked me to give my
answer to the nearest thousandth. So finally I’ll round my answer. And therefore I have
that the length of this chord to the nearest thousandth is 12.257 centimeters.

In this question, we’re given a triangle and we can see that it’s an isosceles triangle
because we’re told that 𝐴𝐵 is equal to 𝐴𝐶. We’re also given some other information
and we’re asked to find the area of this triangle.

First of all then, I’m told about the value of cosine of 𝐵. But there are currently no
right-angled triangles visible within the question. So what I want to do is create a
right-angled triangle.

What I can do, because triangle 𝐴𝐵𝐶 is isosceles, if I draw in the perpendicular
height of this triangle, then it would divide this isosceles triangle into two congruent
right-angled triangles. So that’s filling in this height here. Now that also means that
20 centimeters will be split exactly into two lots of 10 centimeters.

Now I know I’m going to need trigonometry in this problem. So within the right-angled
triangle on the left, I’m just gonna label the three sides in relation to the angle 𝐵.

So we have the opposite, the adjacent, and the hypotenuse. Now I’m told that cos of 𝐵
is equal to five over 13. So I’m also going to need the definition of the cosine ratio.
And remember it’s that the cosine of a general angle 𝜃 is equal to the adjacent over
the hypotenuse.

So let’s think about a strategy for this question. I need the area of triangle 𝐴𝐵𝐶.
Well I know the base is 20 centimeters. So I also need to know the perpendicular height
cause then I can work out the area by doing base multiplied by perpendicular height over
two. So this means I need to work out the length of the opposite side.

I haven’t been given enough information in order to work out the opposite side straight
away because the information about the cosine ratio is about the adjacent and the
hypotenuse.

My strategy then, it’s going to be to work out the length of the hypotenuse using
trigonometry and then use the Pythagorean theorem, which we’ll look at in little bit
more detail later, in order to find the value of this third side.

So let’s start off by writing down the cosine ratio for this triangle then. We have that
the cosine of 𝐵 is equal to 10 over 𝐻. And we also know that this is equal to five
over 13.

So I have here an equation that I can solve in order to work out the value of 𝐻. Now 𝐻
is in the denominator. So I’m gonna multiply both sides of the equation by 𝐻. And I’m
also gonna multiply both sides of the equation by 13 at the same time.

Now when I do that, I get that 130 is equal to five 𝐻. Next I want to divide both sides
of this equation by five. And doing so tells me that 𝐻 is equal to 26. So I’ve got the
length of the hypotenuse of this triangle.

So simply we’re going to use the Pythagorean theorem to work out the length of the third
side. So let’s just recall that. So it’s often referred to as 𝑎 squared plus 𝑏 squared
is equal to 𝑐 squared. But what it tells us is if I take the two shorter sides of a
right-angled triangle, square them, and add them together, then I get the same result as
if I square the longest side, the hypotenuse.

So I’m gonna apply the Pythagorean theorem here. And I’m gonna refer to the opposite side
as 𝑥 rather than 𝘖 just to avoid mixing up with zero. I have then that 𝑥 squared plus
10 squared is equal to 26 squared.

Now I’ve written in the complete working out here. And you can pause the video to look
at it in detail if you want. But essentially, I’m evaluating 10 squared and 26 squared,
subtracting a 100, square rooting, and this tells me that 𝑥 is equal to 24.

So finally then, I have all the information I need in order to find the area of this
triangle. I have the full base of the isosceles triangle, 20 centimeters, and the
perpendicular height, 24 centimeters.

So the area is 20 multiplied by 24 and then divided by two. We have then a final answer
of 240 centimeters squared.

In this question, we have a diagram that involves two right-angled triangles. And we’re
asked to calculate the length of 𝐴𝐶, which is the side in the larger right-angled
triangle.

So first of all, let’s just consider what we’ve got in the smaller right-angled
triangle, triangle 𝐶𝐷𝐸. I’ve been given two lengths. And in the larger right-angled
triangle, triangle 𝐴𝐵𝐶, I have only one length. And it’s the length in that triangle
that ultimately I’m looking to calculate.

Now there is an angle that is common to both of these triangles. And it’s angle 𝐶. So
what my approach is going to be is to calculate angle 𝐶 first using the smaller
right-angled triangle and then combine angle 𝐶 with that length, 19 centimeters, in the
larger triangle in order to work out the length of 𝐴𝐶.

So starting off in that smaller right-angled triangle first, I’m gonna label the three
sides in relation to the angle 𝐶. So we have the opposite, the adjacent, and the
hypotenuse. Now what I can see is that because I have the length of the opposite and the
hypotenuse, I can use the sine ratio in order to calculate this angle 𝐶.

The sine ratio, remember, is defined as the opposite divided by the hypotenuse. So this
tells me that sine of 𝐶 is equal to 11 over 21 in this case. Now I could evaluate angle
𝐶 at this point by using the inverse sine function. And if I did, it would tell me that
angle 𝐶 is equal to 31.6 degrees. But actually that’s not necessary. And you’ll
see why a little bit later in the question.

So now let’s consider the large triangle, triangle 𝐴𝐵𝐶. And, again, I’m gonna label its
three sides in relation to angle 𝐶. So we can see that we have the length of the
opposite side, 19 centimeters. And the length we want to calculate 𝐴𝐶 is the
hypotenuse. So, again, we’re going to be using the sine ratio.

So if I now write down the sine ratio in this larger triangle, well we have then that
sine of this angle 𝐶 is equal to 19 over 𝐴𝐶, that side. So what I can do then is I
can put these two pieces of information together.

As they’re both equal to sin 𝐶, I can then equate them to each other. And this tells me
then that 11 over 21 is equal to 19 over 𝐴𝐶. That’s why I didn’t actually need to
evaluate the angle 𝐶 because it was sin 𝐶 that I used later in the question.

Now I have an equation that I can solve in order to work out the value of 𝐴𝐶. So I’m
gonna multiply both sides of the equation by 𝐴𝐶 and also by 21. So this tells me that
11 lots of 𝐴𝐶 is equal to 19 multiplied by 21. Next I’m gonna divide both sides of the
equation by 11.

So I have that 𝐴𝐶 is equal to 19 multiplied by 21 over 11. And I’ll give my answer for
𝐴𝐶 as a fraction. We have then that 𝐴𝐶 is equal to 399 over 11. And the units for
that are centimeters.

In summary then, we’ve recalled the definition of the three trigonometric ratios sine,
cosine, and tangent. We’ve seen how to identify which ratio is required. And then we’ve
applied them to some mixed problems.