Welch Teaching & Coaching
Trigonometry • College Preparatory Mathematics

Unit 7: Trigonometric Identities

Use fundamental identities to simplify trigonometric expressions and verify identities through valid algebraic transformations.

U7
Unit Road Map

Lesson Map

Use this table as your route through the unit. Lesson numbers jump directly to the matching explanation.

LessonTopicYou should be able to…
7.1Fundamental IdentitiesUse reciprocal, quotient, and Pythagorean identities as equivalent forms.
7.2Simplifying Trigonometric ExpressionsChoose substitutions that reduce an expression to a simpler form.
7.3Verifying IdentitiesTransform one side of an equation until it matches the other side.
Navigation

How the Unit Is Built

Each resource below is part of the same learning cycle. Select a card to jump to it.

Learn

Content Summary

Work through these lessons in order. Focus on the meaning of the mathematics, the decision that selects a method, and a check that confirms the result.

Lesson 7.1

Fundamental Identities

Use reciprocal, quotient, and Pythagorean identities as equivalent forms.

What to understand: Explain what the method is doing, not only which buttons or steps produce an answer.
Check: Ask whether the sign, size, units, quadrant, restrictions, or graph behavior make sense before accepting the result.
Lesson 7.2

Simplifying Trigonometric Expressions

Choose substitutions that reduce an expression to a simpler form.

What to understand: Explain what the method is doing, not only which buttons or steps produce an answer.
Check: Ask whether the sign, size, units, quadrant, restrictions, or graph behavior make sense before accepting the result.
Lesson 7.3

Verifying Identities

Transform one side of an equation until it matches the other side.

What to understand: Explain what the method is doing, not only which buttons or steps produce an answer.
Check: Ask whether the sign, size, units, quadrant, restrictions, or graph behavior make sense before accepting the result.
Watch & Reinforce

Video Library

These links open focused video searches by lesson topic. Use them as reinforcement rather than as a substitute for showing your own reasoning.

Lesson 7.1

Fundamental Identities

Use a short lesson after reading the website explanation, then return to the practice.

Find a matching video →
Lesson 7.2

Simplifying Trigonometric Expressions

Use a short lesson after reading the website explanation, then return to the practice.

Find a matching video →
Lesson 7.3

Verifying Identities

Use a short lesson after reading the website explanation, then return to the practice.

Find a matching video →
Language

Vocabulary

These terms should appear naturally in explanations, diagrams, and written reasoning.

identity
Write a definition in your own words and add an example or sketch in your STRIDES notes.
equivalent expression
Write a definition in your own words and add an example or sketch in your STRIDES notes.
reciprocal identity
Write a definition in your own words and add an example or sketch in your STRIDES notes.
quotient identity
Write a definition in your own words and add an example or sketch in your STRIDES notes.
Pythagorean identity
Write a definition in your own words and add an example or sketch in your STRIDES notes.
verify
Write a definition in your own words and add an example or sketch in your STRIDES notes.
simplify
Write a definition in your own words and add an example or sketch in your STRIDES notes.
Practice

Sample Problems

Try each problem before opening the solution. The examples are original practice aligned to the unit skills.

1

Simplify

Simplify sin x · csc x.
Show solution
csc x=1/sin x, so the product is 1 where defined.
2

Simplify

Simplify tan x · cos x.
Show solution
tan x=sin x/cos x, so the expression simplifies to sin x.
3

Pythagorean

Simplify 1−sin²x.
Show solution
cos²x.
4

Derived identity

Simplify sec²x−tan²x.
Show solution
1.
5

Verify

Verify (1−cos²x)/sin x = sin x.
Show solution
Replace 1−cos²x with sin²x; then sin²x/sin x=sin x where defined.
6

Strategy

Why is converting tangent and secant to sine/cosine sometimes useful?
Show solution
It places expressions over common basic functions and often reveals common factors or Pythagorean identities.
Reference

Equations & Rules

Know what each relationship means and when it applies; do not treat the box as a list of disconnected formulas.

Reciprocal
csc x=1/sin x; sec x=1/cos x; cot x=1/tan x
Quotient
tan x=sin x/cos x; cot x=cos x/sin x
Pythagorean
sin²x+cos²x=1
Derived
1+tan²x=sec²x
Derived
1+cot²x=csc²x
Verification Principle
Work on one side at a time; replace expressions with equivalent expressions rather than assuming the identity is true.
Prepare

Review Guide

Skill Checklist

  • I can use reciprocal, quotient, and Pythagorean identities as equivalent forms.
  • I can choose substitutions that reduce an expression to a simpler form.
  • I can transform one side of an equation until it matches the other side.

How to review

  1. Complete the unit Skills Check packet without notes.
  2. Mark the exact skill behind each error.
  3. Return to that lesson and complete a fresh example.
  4. Explain the correction in words, then retry a similar problem.
  5. Finish with mixed practice so you must choose the method yourself.
Apply

Performance Task

Identity Strategy Guide

Create a visual decision guide that shows when reciprocal, quotient, and Pythagorean identities are useful. Support each branch with an original example and verification.

Evidence to include

A labeled mathematical model, organized calculations, appropriate units and precision, a written interpretation, and a check or discussion of limitations.