### Video Transcript

In this video, we will learn how to
find a missing angle in a right triangle using the appropriate inverse trigonometric
function given two side lengths.

Letโs begin by recapping some of
the vocabulary related to right triangles. Suppose we have a right triangle
like the one shown, with one of the non-right angles labeled as ๐. The hypotenuse of a right triangle
is its longest side, which is always the side directly opposite the right angle. In relation to the angle weโve
labeled as ๐, the side directly opposite this angle is called the opposite. And the side between the right
angle and the angle ๐ is called the adjacent.

Weโll often see the names for these
three sides abbreviated to opp, adj, and hyp, or simply O, A, and H. The three trigonometric ratios
sine, cosine, and tangent describe the ratios between the different pairs of these
side lengths. For a fixed value of ๐, the ratio
between each pair of side lengths is always the same, no matter how big the triangle
is.

We can use the acronym SOHCAHTOA to
help us remember the definitions of the three trigonometric ratios. The first letter in each part
refers to the trigonometric ratio weโre using, either sine, cosine, or tangent. And then the next two letters refer
to the numerator and denominator of the sides involved in the ratio. So SOH tells us that sin of an
angle ๐ is equal to the length of the opposite side divided by the length of the
hypotenuse, O over H. cos of ๐ is equal to the adjacent
over the hypotenuse. And tan of ๐ is equal to the
opposite over the adjacent. We should already be comfortable
with using these three trigonometric ratios to calculate the length of a side in a
right triangle given the length of one of the other two sides and the measure of one
of the non-right angles.

In this video, weโll focus on
finding the measure of an angle given the lengths of two of the triangleโs side. To do this, weโll need to use the
inverse trigonometric functions. These are essentially the functions
that do the opposite of what the sine, cosine, and tangent functions do. We denote these using the
superscript negative one. And we read this as the inverse
sine, inverse cosine, and inverse tangent functions. They are also known as the arcsine,
arccosine, and arctangent functions.

These inverse trigonometric
functions are an alternative way of describing the relationship between an angle and
the values of its three trigonometric ratios. We interpret them as follows. If there is a value ๐ฅ such that ๐ฅ
is equal to sin ๐, then we can equivalently write this as ๐ is equal to the
inverse sin of ๐ฅ. In the same way, if there is a
value ๐ฆ such that ๐ฆ is equal to the cos of ๐, then ๐ is equal to the inverse cos
of ๐ฆ. And if ๐ง is equal to the tan of
๐, then ๐ is equal to the inverse tan of ๐ง.

Now, itโs important to note that
this notation does not mean a reciprocal. The inverse sin of ๐ฅ does not mean
one over sin of ๐ฅ. What this means is that if we know
the value of one of the three trigonometric ratios for an angle ๐, we can work
backward to find the angle associated with this ratio by applying the inverse
trigonometric function. To access these functions on our
calculators, we usually have to press shift and then either the sin, cos, or tan
button to get the inverse of each function.

Letโs look at an example of how we
can use these inverse functions to find the measure of an angle given two side
lengths in a right triangle.

For the given figure, find the
measure of angle ๐ in degrees to two decimal places.

Looking at the figure, we can see
that we have a right triangle, in which ๐ represents the measure of one of the
non-right angles. Weโve also been given the lengths
of two of the sides of the right triangle. They are three and eight units. We can therefore approach this
problem using trigonometry.

Our first step in any problem
involving trigonometry is to label the three sides of the triangle in relation to
the angle ๐. The side directly opposite the
right angle is the hypotenuse, which weโll abbreviate to H. The side directly opposite the
angle ๐ is called the opposite, abbreviated to O. And the side between the angle ๐
and the right angle is the adjacent, which weโll abbreviate to A.

Weโll now recall the acronym
SOHCAHTOA to help us decide which of the three trigonometric ratios we need to use
in this problem. The two side lengths weโve been
given are the adjacent and the hypotenuse. So weโre going to be using the
cosine ratio. The cos of an angle ๐ is defined
to be the length of the adjacent side divided by the length of the hypotenuse. Substituting the values for this
triangle, we have that cos of ๐ is equal to three over eight, or three-eighths.

Now we need to find the value of
๐, which means we need to apply the inverse cosine function. This is the function that
essentially does the opposite of the cosine function. It says if cos of ๐ is equal to
three-eighths, then what is ๐? We have ๐ is equal to the inverse
cos of three-eighths.

We can then evaluate this on our
calculators, making sure theyโre in degree mode. To access the inverse cosine
function, we usually need to press shift and then the cos button on our
calculators. Evaluating gives 67.975. And then we round to two decimal
places as specified in the question. So, by applying the inverse cosine
function in this right triangle, we found that the measure of angle ๐ to two
decimal places is 67.98 degrees.

Letโs now consider another example
in which weโll find the measures of two angles in a right triangle.

For the given figure, find the
measures of angle ๐ด๐ถ๐ต and angle ๐ต๐ด๐ถ in degrees to two decimal places.

Weโve been given a right triangle
in which we know the lengths of two of its sides. We can therefore approach this
problem using trigonometry. Our first step in a problem like
this is to label the sides of the triangle. But in order to do this, we need to
know which angle weโre labeling the sides in relation to. Letโs calculate angle ๐ด๐ถ๐ต first,
and weโll label this on our diagram as angle ๐ฅ. The hypotenuse of a right triangle
is always the same. Itโs the side directly opposite the
right angle. The opposite is the side directly
opposite the side weโre interested in. So the side opposite angle ๐ฅ is
the side ๐ด๐ต. And finally, the adjacent is the
side between our angle and the right angle. Itโs the side ๐ต๐ถ.

We can now recall the acronym
SOHCAHTOA to help us decide which of the three trigonometric ratios โ sine, cosine,
or tangent โ we need to use to answer this question. The sides whose lengths weโre given
are the opposite and adjacent. So weโre going to be using the tan
ratio. For an angle ๐ in a right
triangle, this is defined as the length of the opposite divided by the length of the
adjacent. Substituting ๐ฅ for the angle ๐,
four for the opposite, and five for the adjacent, we have the equation tan of ๐ฅ is
equal to four-fifths.

To determine the value of ๐ฅ, we
need to apply the inverse tangent function, which says if tan of ๐ฅ is equal to
four-fifths, then what is ๐ฅ? Evaluating this on our calculators,
making sure theyโre in degree mode, gives 38.659. We can round this to two decimal
places, giving 38.66. So we found the measure of our
first angle.

To calculate the final angle in the
triangle, we have a choice of methods. We could use trigonometry again, or
we could use the fact that the angles in a triangle sum to 180 degrees. Itโs more efficient to use the
second method. So we have that the measure of
angle ๐ต๐ด๐ถ is equal to 180 degrees minus 90 degrees for the right angle minus
38.66 degrees for angle ๐ด๐ถ๐ต, which is equal to 51.34 degrees. So we found the measures of the two
angles.

If we did want to use trigonometry,
weโd have to relabel the sides of the triangle in relation to this angle, which
means the opposite and adjacent sides would swap round. Weโd still be using the tangent
ratio, but this time weโd have tan of ๐ฆ is equal to five over four. Weโd then have ๐ฆ is equal to the
inverse tan of five over four, which is indeed equal to 51.34 when rounded to two
decimal places.

Our answer to the problem then is
that the measure of angle ๐ด๐ถ๐ต is 38.66 degrees and the measure of angle ๐ต๐ด๐ถ is
51.34 degrees, each rounded to two decimal places.

In the two problems weโve
considered so far, weโve been given a diagram of the right triangle weโre working
with. However, in some trigonometry
questions, we wonโt be given a diagram. And part of the skill of answering
the question is drawing an appropriate diagram ourselves. Letโs now consider an example of
this.

๐ด๐ต๐ถ is a right triangle at ๐ต,
where ๐ต๐ถ equals 10 centimeters and ๐ด๐ถ equals 18 centimeters. Find the length ๐ด๐ต, giving the
answer to the nearest centimeter, and the measures of angles ๐ด and ๐ถ, giving the
answer to the nearest degree.

We havenโt been given a diagram for
this problem. So we need to begin by drawing one
ourselves. Weโre told that ๐ด๐ต๐ถ is a right
triangle at ๐ต, which means itโs angle ๐ต that is the right angle. Weโre also told that ๐ต๐ถ is 10
centimeters and ๐ด๐ถ is 18 centimeters. We were asked to find the length
๐ด๐ต and the measures of each of the other two angles in this triangle.

Letโs begin with finding the length
of ๐ด๐ต. As we have a right triangle in
which we know two of its side lengths, we can apply the Pythagorean theorem to
calculate the length of the third side. The Pythagorean theorem tells us
that, in a right triangle, the sum of the squares of the two shorter sides is equal
to the square of the hypotenuse. In our triangle, the two shorter
sides are ๐ด๐ต and ๐ต๐ถ and the hypotenuse is ๐ด๐ถ. So we have the equation ๐ด๐ต
squared plus ๐ต๐ถ squared is equal to ๐ด๐ถ squared.

Substituting 10 for ๐ต๐ถ and 18 for
๐ด๐ถ gives ๐ด๐ต squared plus 10 squared is equal to 18 squared. This simplifies to ๐ด๐ต squared
plus 100 equals 324. So ๐ด๐ต squared is equal to
224. ๐ด๐ต is then equal to the square
root of 224, which is 14.9666, or 15 to the nearest integer. So we found the length of ๐ด๐ต. And now we need to consider
calculating the measures of the two angles. Letโs start with angle ๐ด.

Weโll begin by labeling the three
sides of the triangle in relation to this angle. The side directly opposite, thatโs
๐ต๐ถ, is the opposite. The side between this angle and the
right angle is the adjacent. And the side directly opposite the
right angle is the hypotenuse. We can then recall the acronym
SOHCAHTOA to help us decide which trigonometric ratio to use to calculate this
angle.

As we now know the lengths of all
three sides in the triangle, we could use any of the three ratios. But it makes the most sense to use
the two sides whose lengths weโre originally given in case we made any mistakes when
we calculated the length of ๐ด๐ต. So weโre going to use the sine
ratio. This is defined as sin of ๐ is
equal to the opposite over the hypotenuse. Substituting 10 for the opposite
and 18 for the hypotenuse and using ๐ด to represent the angle at ๐ด, we have sin of
๐ด is equal to 10 over 18. To calculate ๐ด, we need to apply
the inverse sine function, giving ๐ด equals the inverse sin of 10 over 18. Evaluating on our calculators,
which must be in degree mode, we have 33.748, which to the nearest degree is 34.

Finally, we need to calculate the
measure of the third angle in the triangle. As the angles in any triangle sum
to 180 degrees, we can do this by subtracting the measures of the other two angles
from 180 degrees, which gives 56 degrees. And so weโve completed our
solution. The length of ๐ด๐ต to the nearest
centimeter is 15 centimeters. The measure of angle ๐ด and the
measure of angle ๐ถ, each to the nearest degree, are 34 and 56 degrees,
respectively.

Problems involving trigonometry may
also be presented as a story describing a practical situation. When this is the case, we may not
be given a diagram. And part of the skill will be to
produce one ourselves from the information given in the question. Letโs look at one final example of
this.

A five-meter ladder is leaning
against a vertical wall such that its base is two meters from the wall. Work out the angle between the
ladder and the floor, giving your answer to two decimal places.

We havenโt been given a diagram to
accompany this question. So the first step is going to be to
draw one. We have a ladder leaning against a
vertical wall. The triangle formed by the ladder,
the floor, and the wall is a right triangle. And in fact, thatโs all we really
need to draw for our diagram. The ladder is five meters long, and
its base is two meters away from the wall. We are asked to work out the angle
between the ladder and the floor. So thatโs this angle here, which
weโll call ๐ฅ.

We have a right triangle in which
we know the lengths of two sides. And we want to calculate the
measure of an angle. So we can apply trigonometry. We begin by labeling the three
sides of the triangle in relation to the angle ๐ฅ. So we have the opposite, the
adjacent, and the hypotenuse. We then recall the acronym
SOHCAHTOA to help us decide which of the three trigonometric ratios to use in this
problem. The side lengths we know are the
adjacent and the hypotenuse. So weโre going to be using the
cosine ratio. This is defined for an angle ๐ as
the length of the adjacent divided by the length of the hypotenuse. Substituting two for the adjacent
and five for the hypotenuse, we have that cos of ๐ฅ is equal to two-fifths.

To find the value of ๐ฅ, we need to
apply the inverse cosine function, giving ๐ฅ is equal to the inverse cos of
two-fifths. Evaluating this on our calculators,
which must be in degree mode, gives 66.421. Finally, we round our answer to two
decimal places. And we found that the angle between
the ladder and the floor is 66.42 degrees.

Letโs now summarize the key points
that weโve seen in this video. First, we recalled the terminology
associated with the three sides of a right triangle: the opposite, the adjacent, and
the hypotenuse. We then recalled the definitions of
the three trigonometric ratios โ sine, cosine, and tangent โ and the acronym
SOHCAHTOA, which we can use to help remember their definitions.

We then saw that we can find the
measure of an angle by applying the inverse trigonometric functions, which take the
value of one of these three trigonometric ratios and tell us the angle associated
with it. We saw that we can apply these
techniques to a range of problems involving right triangles, including story
problems, which describe a practical situation.