Using Graphs to Determine Trigonometric Identity

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  • 0:01 Trigonometric Identity
  • 0:53 Trigonometric Equations
  • 2:17 Graphing the Equation
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After watching this video lesson, you will be able to use graphs to determine whether an equation is a trigonometric identity or not. Learn what you need to look for to determine this.

Trigonometric Identity

In this video lesson, you will learn what to look for in a graph to determine whether a particular equation is a trigonometric identity or not. But first, let's define trigonometric identity. A trigonometric identity is any trigonometric equation that is always true for all values of the variable. What does this mean?

This means that if you see an equation and you can say for certain that this equation is always true and you can use the equation with confidence, then this trigonometric equation is a trigonometric identity. Remember identity means the same. Think of the related word 'identical.' You can also think of identity as meaning 'self.' Like when someone asks you, 'What is your identity?' You answer with the kind of person you are or you show your identification card, your identity card.

Trigonometric Equations

So, what kinds of equations can you expect to see as we look at trigonometric equations? In math, we already have some trigonometric equations that we know are true. Once we know that a certain trigonometric equation is true, then we can call it a trigonometric identity. For example, these trigonometric equations have been known as trigonometric identities for many, many years:

graph trig identity

We use these trig identities to help us solve more complicated trig problems. For example, if we see sine squared plus cosine squared inside a larger problem, we can substitute 1 for that part of the problem, thus making the problem that much simpler and easier to deal with.

Let's take a look at a trigonometric equation that we have been given:

graph trig identity

Many times, when we are working with trigonometric equations, you will see a variable such as x instead of theta. The letter or symbol that is used to represent our variable doesn't matter. As long as you know which letter is your variable, that is all that matters. So, here x is our variable. So, the question now is how can we show that this trigonometric equation is an identity?

Graphing the Equation

We can do that by graphing each side of our equation. So, what we do is we first graph sin (x) cos (x) tan (x), then we graph 1 - cos^2 (x) to see if they are equal. We can use a graphing calculator to do this or any other graphical method that is easy for you.

Why don't we go ahead and graph each side.

First, the left side, sin (x) cos (x) tan (x). We get this graph:

graph trig identity

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