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## Pythagorean Trigonometric Identities

#### High School Trigonometric Functions - HSF-TF.C.8

What Are Pythagorean Trigonometric Identities? In mathematics, identity is referred to as an equation that stands true for all values A trigonometric identity refers to an equation with trigonometric functions, and that stands true for every value substituted for a variable. Trigonometric identities help in simplifying trigonometric expressions. Trigonometric identities involving the Pythagorean theorem are the most commonly used ones. In the unit circle, i.e., the circle with a radius of 1, a point on a unit circle (vertex of a right triangle) can be represented by cos(θ) and sin (θ). Now, the adjacent and opposite of right triangle has values of sin(θ) and cos(θ), the Pythagorean theorem can be applied to obtain Sin2(θ) + cos2(θ) = 1 This equation is known as first Pythagorean identity. It stands true for all values of theta in a unit circle By using the first Pythagorean identity, we can obtain other identities Sin2(θ) + cos2(θ) = 1 Dividing each term by cos2(θ) Sin2(θ)/cos2(θ) +cos2(θ)/cos2(θ) = 1/cos2(θ) We know 1/cos(θ)=sec(θ) and sin(θ)/cos(θ)= tan(θ) Simplifying we term, we get : Tan2(θ) + 1= sec2(θ) We now have our second Pythagorean identity : Tan2(θ) + 1 =sec2(θ) Using the first identity to obtain the third Pythagorean identity : Sin2(θ) + cos2(θ) = 1 Dividing each term by sin2(θ) Sin2(θ)/sin2(θ) + cos2(θ)/sin2(θ) = 1/sin2(θ) We know that 1/sin(θ) =cosec(θ) and cos(θ)/sin(θ) =cot(θ) 1 + cot2(θ) = cosec2(θ) The third Pythagorean identity is : 1 + cot2(θ) = cosec2(θ) These worksheets and lesson can help you solve and better understand the most common trigonometric identities.

### Printable Worksheets And Lessons  #### Homework Sheets

These problems are all over the concept brought up by the standard area.

• Homework 1 - You will need to match the known identities to expressions.
• Homework 2 - These can be worked through in a number of different ways.
• Homework 3 - Use the given triangle to help you solve the problem.

#### Practice Worksheets

This section covers the majority of values that you will see on national exams.

• Practice 1 - Simplify the expression: Sec2 x – cot x tan x to a single trigonometric function.
• Practice 2 - If cos Θ = 8/14 Find the values of the cot Θ, using a Pythagorean identity.
• Practice 3 - Use a well known Pythagorean identity to solve this.

#### Math Skill Quizzes

You really need to spend sometime reviewing the trig. Identities before tackling these.

• Quiz 1 - You should know be able to find these values quickly.
• Quiz 2 - What is the missing piece?
• Quiz 3 - How can you solve each of these with the fewest steps?