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Trigonometric functions are often used to perform calculations in real-life fields, such as architecture, optics, and trajectories. In many cases, multiple trigonometric functions appear in one expression, potentially causing confusion. Fortunately, rules that relate different trigonometric functions exist, providing the ability to simplify calculations. This lesson will present some of them.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

Firework's Height Given by Trigonometric Functions

Best friends Paulina and Maya graduated high school last Friday. At the graduation party, they, along with the rest of the graduates and guests, were admiring bright and colorful fireworks.
The fireworks at the graduation party
External credits: @pch.vector
Their physics teacher came up with a contest to play for a prize — three tickets to an Ali Styles concert! He said that the height of the firework and horizontal displacement were related by the following equation.
Here, is the initial velocity of the projectile, is the angle at which it was fired, and is the acceleration due to gravity. The first student, who can rewrite the equation so that is the only trigonometric function, will win the tickets.
Discussion

The Concept of a Trigonometric Identity and Some Examples

Consider the two given equalities. First, analyze if any of them can be simplified. Then, focus on how many values of make each of them true. What is the main difference between the equalities?

Two equalities: I. x^2+3x=4 and II. x^4/x^3=x

The first equality is an equation that can be solved to find the few, if any, values of that make the equation true. However, the second equality can be simplified to and, therefore, is an equation that is true for all values of for which the expressions in the equation are defined. Given that characteristic, the second equality is called an identity.

A trigonometric identity is an equation involving trigonometric functions that is true for all values for which every expression in the equation is defined.

Two of the most basic trigonometric identities are tangent and cotangent Identities. These identities relate tangent and cotangent to sine and cosine.

Rule

Tangent and Cotangent Identities

The tangent of an angle can be expressed as the ratio of the sine of to the cosine of

Similarly, the cotangent of can be expressed as the ratio of the cosine of to the sine of

Proof

Tangent Identity

Two proofs will be written for this identity, one using a right triangle and the other using a unit circle.

Right Triangle

In a right triangle, the tangent of an angle is defined as the ratio of the length of the opposite side to the length of the adjacent side

Right triangle with side lengths k, l, and m and acute angle theta
At the same time, the sine and cosine of can be written as follows.
By manipulating the right-hand side of the equation the tangent can be expressed as the sine over the cosine of
The proof of the identity is complete.

Unit Circle

Consider a unit circle and an angle in standard position.

unit circle

It is known that the point of intersection of the terminal side of the angle and the unit circle has coordinates

point of intersection

Draw a right triangle using the origin and as two of its vertices. The length of the hypotenuse is and the lengths of the legs are and

right triangle
As shown previously, the tangent of a right triangle is defined as the ratio of the length of the opposite side — in this case, — to the length of the adjacent side, which here is

Proof

Cotangent Identity

Two more proofs will be written for this identity, one of them using just a right triangle and the other using a unit circle.

Right Triangle

In a right triangle, the cotangent of an angle is defined as the ratio of the length of the adjacent side to the length of the opposite side

Right triangle with side lengths k, l, and m and acute angle theta
Additionally, the sine and cosine of can be written as follows.
By manipulating the right-hand side of the equation the cotangent can be expressed as the cosine over the sine of
This proof is complete.

Unit Circle

Using a unit circle, it has been already proven that the tangent of an angle is the ratio of the sine to the cosine of the angle
By manipulating the above equation, it can be shown that the cotangent of is the ratio of the cosine of to the sine of

This proof is complete.

There are also trigonometric identities which show that some trigonometric functions are reciprocals of others.

Rule

Reciprocal Identities

The trigonometric ratios cosecant, secant, and cotangent are reciprocals of sine, cosine, and tangent, respectively.

Proof

Consider a right triangle with the three sides labeled with respect to an acute angle

A right triangle ABC with the angle theta at the vertex B
Next, the sine, cosine, tangent, cosecant, secant, and cotangent ratios are written.
The reciprocal of the sine ratio will now be calculated.
Solve for
It has been found that which is the reciprocal of is equal to By the definition, the cosecant of is also the ratio of the lengths of the hypotenuse and the opposite side to Therefore, by the Transitive Property of Equality, is equal to
By following a similar procedure, the other two identities for secant and cotangent can be proven.
Example

Simplifying Expressions by Using Trigonometric Identities

Thinking of different ways to solve the firework challenge, Paulina found herself thinking about her time learning trigonometric identities earlier in the school year. She really enjoyed those lessons.

A notebook with a pencil

A few of her favorite exercises included the following where she was asked to simplify these expressions.

a
b
c

Hint

a Recall the Tangent Identity.
c Start by simplifying the first term of the sum. Apply the Reciprocal Identity for cosecant and the Tangent Identity.

Solution

a The first expression contains two different trigonometric functionssine and tangent. This means the Tangent Identity could be useful here.
Substitute for into the expression and simplify.
Reduce by
b To simplify the second expression, recall the Cotangent Identity and the Reciprocal Identity for secant.
Next, substitute the expressions for and and simplify.
Note that the obtained expression is actually equal to
Therefore, the last expression simplifies to
c The last expression is the sum of two terms.
Begin by simplifying the In order to do this, review the Reciprocal Identity for cosecant and the Tangent Identity.
Next, use these identities to find a simpler form of the first term.
By the Reciprocal Identity for secant, the obtained expression is equal to
This means that the first term can be simplified to
Therefore, is the most simplified form of the given expression.
Discussion

Suggestions for Verifying Identities

Some techniques, like the following, are helpful when verifying if trigonometric identities are true.

  • Substitute one or more basic trigonometric identities to simplify the expression.
  • Factor or multiply as necessary. Sometimes it is necessary to multiply both the numerator and denominator by the same trigonometric expression.
  • Write each side of the identity in terms of sine and cosine only. Then simplify each side as much as possible.
Verifying an identity tan(theta)=sec(theta)/csc(theta)
Remember that the properties of equality do not apply to identities as they do with equations. It is not possible to perform operations to the quantities on each side of an unverified identity.
Discussion

Presenting Pythagorean Identities

One of the most known trigonometric identities relates the square of sine and cosine of any angle This identity can be manipulated to obtain two more identities involving other trigonometric functions.

Rule

Pythagorean Identities

For any angle the following trigonometric identities hold true.

Proof

For Acute Angles
Consider a right triangle with a hypotenuse of
right triangle with hypotenuse 1
By recalling the sine and cosine ratios, the lengths of the opposite and adjacent sides to can be expressed in terms of the angle.
Definition Substitute Simplify

It can be seen that if the hypotenuse of a right triangle is the sine of an acute angle is equal to the length of its opposite side. Similarly, the cosine of the angle is equal to the length of its adjacent side.

Right triangle with hypotenuse 1

By the Pythagorean Theorem, the sum of the squares of the legs of a right triangle is equal to the square of the hypotenuse. Therefore, for the above triangle, the sum of the squares of and is equal to the square of

Since represents a side length, it is not Therefore, by diving both sides of the above equation by the second identity can be obtained.
Simplify

The second identity was obtained.
Since represents a side length, it is not Therefore, by dividing both sides of by the third identity can be proven.
Simplify

Finally, the third identity was obtained.


Proof

For Any Angle

The first identity can be shown using the unit circle and the Pythagorean Theorem. Consider a point on the unit circle in the first quadrant, corresponding to the angle A right triangle can be constructed with

A right triangle with an angle theta on a unit circle
By the Pythagorean Theorem, the sum of the squares of and equals
In fact, this is true not only for points in the first quadrant, but for every point on the unit circle. Recall that, for points on the unit circle corresponding to angle it is known that and that By substituting these expressions into the equation, the first identity can be obtained.
Dividing both sides by either or leads to two variations of the Pythagorean Identity.
Second and third identities are derived from the first one
Now it will be shown how the Pythagorean Identities can be applied to solve different mathematical problems.
Example

Trigonometric Functions of Angles on the Map

Enjoying the graduation party, Maya and Paulina shared some memories about some of the fun they had exploring their neighborhood as kids. After school they would buy junk food at the hour store, bird watch in the trees of the park, and play around the lake.
The map of Paulina's and Maya's neighborhood
External credits: Hari Panicker
The sine of the angle formed by the road connecting Paulina's and Maya's houses to the store is The cosine of the angle formed by the road connecting Paulina's and Maya's houses to the school is Interact with the map to view these angles.
a What is the cosine of
b What is the tangent of
c What is the sine of
d What is the cotangent of

Hint

a Apply the Pythagorean Identity.
b Recall the Tangent Identity.
c To determine the sign of the sine, identify in which quadrant is located.
d Use the Cotangent Identity.

Solution

a It is given that the sine of is To find the value of the cosine of the Pythagorean Identity can be used.
Substitute the known value of and solve for
Solve for
To determine the sign of identify in which quadrant the angle is situated.
The map of Paulina's and Maya's neighborhood with the x- and y-axes and quadrants identified
External credits: Hari Panicker

The angle is in the first quadrant, where cosine is positive. Therefore, it can be concluded that the cosine of is

b To calculate the tangent of the Tangent Identity can be used.
Since both and are known, substitute their values and solve for
Simplify right-hand side
c This time the sine of the angle is known, while the cosine should be found. Again, the Pythagorean Theorem can be used. Substitute for and solve for
Solve for